Also, the firms who take the other side of that bet talk, and you aren't going to be able to get tens of billions of dollars of derivatives without people figuring out what you're doing and acting against you.
You can make those bets on regulated exchanges, thanks to bitcoin-trust ETFs. IBIT is the largset of those, with $74bn assets under management, and it looks like options contracts are available. Correlated bets would also be easily available with companies like Microstrategy.
I think the biggest risk to such a move would be legal, since executing a demonstration 51% attack would plausibly result in a market manipulation investigation.