> We all don't use mainframes because marginal or downright dodgy business cases of our products simply won't pay for it, thus we are stuck in this race to the bottom.

Not just because of the expensive hardware, but also because it's all but impossible to get your hands on them as a developer. With Java, JS, Python, .NET, whatever companies have an insanely large pool of people to choose from... quite a few self-taught, some who went to university and studied CS, some who went to more or less decent bootcamps, some who "grew into" programming from other roles. On top of that, last I heard (admittedly a decade ago) the tooling to work on mainframe systems is just as old as the code they're running, so no modern IDEs, debuggers and whatnot.

With mainframes however, companies have to spend their own money, and quite a lot of it, to get trained developers, and on top of that pay a hefty premium for those who submit to fossilized tooling willingly.