Two Things. First a response with some info for Wjfox and second some points of contention I have with Starspawn.
I'm not up to date on the latest marxist theory but I'm know Piketty has been critiqued to hell and back and I'm almost positive some of his findings are empirically and or argumentatively incorrect. I can't point to specifics, and some of his arguments are sound. This is just nascent knowledge I have from reading various left pieces over the past few years as Piketty's work is very discussed. Growth is slowing though, that central point is true but there's more nuance that I don't have sound knowledge of but know exists.
Micheal Roberts a marxist economist in the City of London has a blog about his work and the tendency of the rate of profit to fall (TRPF.) Overall the rate of profit has been falling since the industrial revolution on average. The end result of this trend is the end of capitalism which is in line with classical marxist economics. Capitalism depends on growth to survive, when there is no profit to be had businesses will be forced towards more desperate and unstable methods to extract profit from the economy. We already saw the start of this with neoliberalisms affinity for austerity. When profit is hard to find gutting public programs can provide temporary relief as can unstable debt practices. Expect capital to be more volatile and desperate as profit continues to fall. The same concept applies to growth.
https://thenextreces...ofit-1948-2015/ (An example of how the US's profit rate has fallen)
This trend is excluding the climate crisis which left unchecked will cause collapse in production yields, profit rates and ultimately growth. The projections above I feel might not be fully accounting for different climate crisis scenarios. In the event of unchecked climate change (current path) the projection above would look more precipitous (still gradual but rapid) with growth easily falling into negative rates by the 2070s. Billions of people dying in a runaway climate scenario cannot maintain a growth economy, it defies reason to think that such a circumstance could. Even in a more managed scenario I imagine growth rates will shrink to well under 1% by 2070 given that growth is fundamentally opposed to environmentalism.
To answer your question more directly. I think that by the latter half of the century capitalism will have collapsed in most countries with only a few enclaves remaining globally. Other economies will have either developed some new economic model that can cope with a contracting economy (maybe socialism maybe something new) or they will have degenerated into neofuedal/monopolized oligarchies as what wealth is left becomes increasingly stratified. Post 2100 assuming we survive the climate crisis I expect humanity to find new growth in exploitation of the solar system.
I don't think feudal societies are capable of coping with the climate crisis due to the inherent instability and strife of constant class conflict within such societies. As such, when humanity starts growing again both feudalism and capitalism will be thoroughly dead excluding the fringes of civilization. This means that whatever form such a new growth economy will take it will not look like anything that has come before. If we don't manage the climate crisis it is very likely humanity fluctuates between feudal, mercantile, tribal and slave economies on a desert planet for what may as well be the rest of time. The reason being that surface fossil fuels have already been mostly extracted meaning that another industrial revolution based on more primitive methods is probably impossible. A future industrial revolution could burn biomatter but such a future civilization would burn out rapidly due to mass deforestation.
Workers do not sell their labour, it is extorted from them. By definition one must have negotiating power to influence price in order to sell something. When a price is imposed there is no agency and hence no sale. This is not sale, it is theft, or more strongly worded a form of slavery. Businesses collude to fix wages and this collusion is more pronounced the farther down the income ladder one goes. How does this happen? A capitalist and or a business both command capital. When they decide a price is too high they have the luxury of refusing to buy and can sit on their wealth. When enough people with capital refuse to buy something of value that thing lowers in price in concordance with supply and demand.
Why is this different for the worker? Why are prices fixed for the worker and hence unfair? Because the worker does not have capital, they only have their labour power. Unlike the capitalist they are compelled to sell their labour power or else they will starve. This in accordance with supply and demand draws prices down for labour as workers scramble to sell in accordance with supply and demand. This is also why lower income workers have less agency compared to higher income workers. Higher skilled and payed workers have more capital to sit on and demand to draw from. Market forces are not even on the side of the worker in the case of switching employers because there is such a glut of labour available on the market and no for-profit business is going to pay above market rate out of the good of their heart.
So what ends up happening is that the lowest payed workers race each other to the bottom and progressively accept less and less pay while those with capital get to completely control the price of the labour due to their economic leverage despite not producing such labour, hence, price fixing, collusion and exploitation. Where these monied people do not produce value, nor negotiate fair terms for such value. They steal the value from desperate people who have no choice and in a fairer economy would command much higher prices.
Let us describe the case of the slave. The slave is given what their master affords them and does not have agency in such matters. The case of the worker is identical with one "difference." The worker is given what their boss affords them and does not have agency in such matters. They can however choose to work for a different boss. But again because they have no agency another boss will functionally be the same as their last and not treat them any better economically. Functionally there is no difference. This is how the condition of the worker and slave are the same.
When unions enter the equation workers gain economic leverage of course. But most modern unions are not generally militant and don't afford economic leverage to their workers. The reason this condition exists at all is because the state regulates how badly businesses (masters) are allowed to treat their workers (slaves.) The state also regulates how a union can act and uses military force to enforce such a condition. State regulation prevents businesses from putting their workers in too harsh of conditions (in the west,) or paying them so little they cannot eat or sleep at all. Doing this prevents real militant unions from forming which are historical analogues to slave revolts. State regulation of workplaces is akin to laws that force slave owners to treating their slaves better so that the slaves don't revolt. That was a rant, but it's late so I'm rambly and it's true.
Second point I took issue with and I will be brief.
Most things are not nearly free to the western consumer if you don't believe me try living on 1,100$ a month in any American city for a little while, it's not comfortable. You'll quickly find that rent eats at least 500$, food another 100-200$, clothing and hygiene 50-100$ depending on the month, utilities 100-200$, transport 75-300$ and medical care more than the entire yearly budget. If things were nearly free than so many people would not be struggling like this. This is less of an academic argument (although one of those could be easily made but do any of us want another 5 paragraph tired Erowind rant?) and more of an appeal to the objective reality of millions of people.