How would it go down? We'll never know the extent of the damage but I think it won't effectively kill Bitcoin. These are all theoretical talks. The only available actual references of successful 51% attacks are those on certain altcoins which are incomparable to Bitcoin anyway. And they're still around. Bitcoin Gold, Ethereum Classic, Grin,
From me you got it right, but i don't really know for any other once, and it's obvious that before something been into attack they most be a competition which @Darker45 stated above, so the competition of numerous coins can bring total reduction to the existing one, so the point is very clear that what might cause the 51% of attack might be the results of alternative coin's.
See that's a different subject, 51% & double-spending, aka ordinary theft one time, or a complete destruction of bitcoin, sort of clean-bomb on crypto.
The wealth/power required to play 51% of btc, well if you had that kind of wealth/power why would you waste it on a teenage prank. On the other hand say bringing the crypto system to its knees the weak link is USD-T, as nobody plays with real money, and for all the talk about about bitcoin being fungible, if&when USDT implodes so does all crypto including bitcoin.
USDT is centralized, so perhaps a hacker/extortion scam, or just an old fashion high-explosive takedown of their infrastructure. There is always an achilles heal in every fragile system, while bitcoin, can be defended on principal of dispersion, its trade on the back of USDT makes it humpty-dumpty, should the great fall begat.
Like has already been said, if your goal is a quick $200M USD, then best to go after a 51% attack on an alt, can be done on a realistic budget. If your goal is to bring down say GOV financial system, then that requires different kind of thinking.
James Garwin, nuke advisor for six presidents said it best years ago, "The most troubling decision to drop the atomic bomb on Japan, was that we could no longer deny its existence", see the cat would be out of the bag. Most things people think they know, aren't even true. What's possible is actually easier than they tell you. The real problem, is that that the 'question' isn't defined.
Perhaps the OP doesn't really even know what 51% attack means? It was what made Satoshis 2009 work new, as opposed to the prior imf-bis white papers of 1997 ("how to make a digital bank"), the solution to the double spending problem. But in the context here I doubt that the OP means 'who can double spend btc', I think its more like who can attack bitcoin? Then you need to step back and ask the question, what is the achilles heal of bitcon.
Just like the USA in time BTC is going down, there is no particular rush, its all controlled demolition. Just like the US-Dollar its going down. BTC is dependent upon SHA256&SECP256k1 both NSA algo's with a limited shelf-life ( 10-15 years from release )
Today in CHINA, where everybody wants to be rich, the only game in town is 'crypto' so everybody plays, and nobody wants to kill the golden-goose, in the USA where everybody is a meth-head living on the streets covered in their own urine, I doubt they give BTC much thought. In summary for now, those that actually control bitcoin (the Chinese), love it much more than the paid influencers found on this site.