Valve has already proven that one moderately-sized corporate contributor added to your moon-shot FOSS project can get you places. (And Valve isn't anywhere near in size for the FANG+ corps you listed)

but why would any of those companies want to use ReactOS? They already build on top of Linux, except maybe Microsoft, but certainly Microsoft wouldn't want to fund ReactOS...

If I were an executive at those places and somebody proposed ReactOS to replace our foundation, I'd assume they were joking/trolling and would laugh (and genuinely find it funny)

There's still a ton of critical, often custom-written (read: hard to replace), corporate Windows software out there that still needs to run. If Microsoft continues their further decline of Microsoft Windows releases then a free alternative starts to look more attractive. Linux+Wine is definitely a good option today but compatibility shims add complexity and not all shops run Linux or want to.

i just want to program my old motorola radios without having to deal with virtualbox windows 98 VMs, you know? one can transcribe "motorola radio" for "bosch end mill from 1993" or "thermoplastic former from 1999"

who said they need to use reactOS or "replace their foundation", which is a term i cannot parse, i don't think anyone wants to use this as a server platform (that was my default assumption of that phrase.)

this is "we have a market cap of 61,271,506 times the median household income in the USA, we can afford to peel off 1/500,000th (0.0002%) of our market cap per year to make this project awesome because we like this project and want to see it grow"

Well if that's the case, then what you're arguing for is charity. That's fine, but the upthread topic has been about business case. If we're looking at it as charity and not a business investment, then it's a much different conversation. I'd certainly love if companies did things like this and would highly applaud it.