Of course they stay on blackchain. And with time when computers will get stronger, they will be companies, that will search for this long lost wallets. Same as are now companies, that search fro sunken ships from oceans. Pulling them out and sell their Gold or other things to pay costs and divide profit. So some coins on today lost wallets are not lost forever.
I think it would be disastrous if it ever happens, it would simply mean that wallet are not secure enough and people confidence in bitcoin would drop. If a vulnerability is found, we can just hardfork and go on a new algorithm and the old wallets would not be cracked.
Don't sweat it. Febo hasn't done the necessary calculations to determine how profitable such an endeavor might be.
Protip: barring some breakthrough in cryptography, our computers will never be powerful nor plentiful enough to make such an endeavor profitable.