Controlling diabetes is vastly more difficult than simply rolling up your sleeves to get a vaccine. COVID vaccines are now widely available, free, safe, and simple. I don't think it is a fair analogy.
I don't think hospitals can ethically refuse to care for the unvaccinated (as personally tempting as that might be), but I do think that insurers (famously not beholden to morality) could/should consider not paying for that care, or at least having a different scale of coverage for the unvaccinated and stupid.