"Highlight 1: That's not a death-sentence. Lands could be artificially raised. Barriers could be erected. Seasteading could get the attention and effort it deserves. We're supposed to be futurists; let's not be doomers. Necessity is the hermaphroditic parent of invention."
I'm not being a doomer. What I'm saying is in congruence with scientific findings and logistical limitations. The emissions cost required to build a dam across the 500+km stretch of the Bay of Bengal to stop the flood is far too great. We don't have the emissions budget to seastead 100million+ people. We can't raise the ground of entire countries. That's not to mention that our capitalist world would never offer pay for any of this even if we could. The European Union won't even build a sea wall in the North Sea how are countries with significantly less purchasing power like Bangladesh going to engage in megaprojects of an even grander scale? Such a project would need to functionally begin tomorrow and the water is already coming.
Seawater will creep around any smaller barriers individual communities and polities erect. The only option is mitigation, relocation and combat to slow and one day reverse climate change.
Since the 1990s the sea level has risen between 0.1-0.13 inches per year. This process is accelerating. Taking an unrealistically conservative (yet still liberal by many denialist standards) expectation that the sea will only rise 9ft by the end of the century roughly half of the country including the capital city of Dhaka will be underwater. This figure is found by multiplying a conservative ~0.11in yearly rise by 80 years. The only thing I can guarantee is that the image below is probably wrong. That the impact will likely be much worse as climate models incorporate more feedback loops over the years and runaway ice sheet loss takes hold.
"Highlight 3: They're also well-positioned to get their daily dose of climate refugees and newly-thawed primeval pathogens. But hey, everything has downsides, doesn't it?"
Climate refugees may be a downside. It depends on how a future Russia handles the situation. They could incorporate them and offer them work building a new world in Russia. A lot of hands are going to be needed to tend all those future Siberian farms that are going to be feeding all of us. Hopefully by the time pathogens like that become a threat the world will have learned its lesson from the coronavirus and properly contain them. If not, Russia won't be any worse off than the rest of us.
In any case I agree with your sentiment on looking for solutions and inventing new ones. Many of these cities that will be under water could become Venices of the future (with somewhat reduced populations) if the the foundations of their many skyscrapers get reinforced before the tides rise. This would help alleviate some of the need for relocation. New pedestrian streets may be built above the old ones in sufficiently dense urban areas. Skyscrapers are also very resilient buildings, not even hurricanes can take them out when their engineering is done right.
I agree too that this thread is about more than Russia by extension. I've also been in many a thread that spiral into off topic discussion. Tis a balance :-)