Unhealth among dogs is more an issue with the fact that unhealthy traits have been selectively bred for over the centuries.
For example, dachshunds' spines sag bc they lack structural integrity, pugs' nasal pathways are too scrunched to allow proper airflow so they're always choking, even german shephards are becoming increasingly barrel-chested. The purse dogs you describe are generally colder bc their little bodies don't retain heat well, and are more subject to infections as a result.
It's for the same reason that the Hapsburgs developed deformities and health problems after a few generations. Imagine if humans were bred the same way we breed dogs. Mandatory inbreeding
There are reasons that mutts are always healthier and longer-lived and that's because their parents were of a diverse background.