One of the major things that has always bothered me about crypto: if an economically "irrational" large player wanted to 51% something like Bitcoin, they could.

I am thinking of, for example, a nation-state. Let's say the US, EU, or China decided for some reason that it was in their national interest to blow up Bitcoin. This could happen if an adversary like Russia or its allies were using Bitcoin for funding and there was a war or a major Cold War style struggle. Such players could afford to purchase and build, in secret, a huge mining farm, and then suddenly turn it on, not caring about the cost because the goals are strategic. It would be massively expensive but it doesn't matter for this case.

A more economical version of the same thing is to engage in honest mining through several front companies that together have 51%. Until a strategic opportunity presents itself and they start colluding.

While that's certainly possible with a large enough expenditure, they'd also have to have the miners be sufficiently indistinguishable that they couldn't easily be denylisted with an update to the official codebase.

State entities can also destroy real banks with all sorts of means if they really want. The vulnerability is real, but beyond the scope of discussion because then it's war we're talking about.

I'm also curious about an attack vector whereby if a coin has a single reasonably well-installed mining software stack, this effectively gives the developers of that stack control over any miner, which could easily add up to 51% if there's only a few mining software options. Sneaking in a backdoor is well within the capabilities of any developer; do the mining companies compile from source?

The moment anyone does this, people will notice, and the coin plummets.