That's not what happens with altcoins that get 51% attacks though. Also, some exploits like that can become visible on the chain but others can't. For example, if someone cracks SHA256 and starts spending from some addresses, there will be no evidence on the chain for this exploit and the owners may not even find out in days or weeks if the exploiter goes after addresses that have not been touched for a long time. And even if the owners find out, their first guess will be someone has cracked into their computer or wallet. They won't even dream of SHA256 hash getting cracked. It will take weeks/months before the community finds out what's going on, depending on how aggressive the exploiter acts. But even a few good hours will be enough for the exploiter to cash out a good sum of money, so things like bitcoin will always be worth exploiting.