The US government lets a lot of unconscionable things happen. How can we move things like Medicaid reimbursing enough (or even as much as Medicare) so that more healthcare providers accept it so more poor people get healthcare up the priority list, to near where “not letting startups collapse” might be?

Why? They took on direct financial risk. They are "sophisticated" financial actors unlike the general public, with access to professional advice on managing their risk profile.

I think it would be better for the economy and the long-term prospect for American innovation to ensure a robust start-up sector, and thus support the current plan to cover the banks deposits (and probably make a profit at doing so, so won't even cost taxpayers). But I don't think these companies have any sort of "moral right" to their deposits against the interests of the rest of society.

Why not round-robin the dollars, so the people with the most uninsured cash take most of the hit?