A 51% attack mainly lets you double spend, so you have to: (1) get a lot of coins and spend them on something (2) start mining a fork where that spending didn't happen (but other real transactions continue to happen) --- your hash power will now need to be dedicated to this new fork at the exclusion of the old "real" fork since all future hashes will diverge (3) wait until the thing you paid for with coins can no longer be "taken back", all while continuing efforts on the alternate fork (4) do the 51% attack to swap out the old "real" fork with your new fork where you never bought the thing, thus keeping the thing _and_ the coins you spent on them (5) all this needs to be balanced with the opportunity cost had you just continued mining on the real chain...
Such an attack would trigger immediate attention since at (2) the main chain would suddenly see a drop in contributions. Also realistically most things you can purchase with coins (like fiat or other goods/services) can be relatively easily "taken back" unless a considerable amount of time is spent to launder them --- say if you "buy" a car with crypto and reverse the transaction, what you've really done is just stolen a car with a lot of extra steps... Which means your only real viable path is to redeem for some other cryptocurrency... Now given that a big attack just happened on the largest market cap crypto, there's a pretty good chance the other coins are going to take a hit and exchanges might even temporarily limit activities while this situation is "investigated", greatly reducing what you can actually do at the end of the day.
This kind of thing is more reasonable on the smaller market cap "meme" coins where you can just exchange them for for a mass-market crypto.