what if everyone can trivially sign a message showing he is the (suppoesedly) rightful owner of an asset in that previous block (or earlier)?
That is what would happen if there is a huge security hole (like private keys were generated by a broken RNG… This has actually partially happened in bitcoin [not in a grand scale, but for individual users with some wallets], some coins have been stolen and others given back by white hat hackers like /r/joehoe, … It could be given back, because the errors were either in RNG or similar, and the crypto used was not fundamentally broken. But the day this would happen, a battle tested crypto completely broken overnight… which is very seldomly the case, then no way to know who was the rightful owner… if anyone can sign an equal valid message).
But yes you are right, in other cases that are more likely to happen, things would not break for everybody all at once
That is what would happen if there is a huge security hole (like private keys were generated by a broken RNG… This has actually partially happened in bitcoin [not in a grand scale, but for individual users with some wallets], some coins have been stolen and others given back by white hat hackers like /r/joehoe, … It could be given back, because the errors were either in RNG or similar, and the crypto used was not fundamentally broken. But the day this would happen, a battle tested crypto completely broken overnight… which is very seldomly the case, then no way to know who was the rightful owner… if anyone can sign an equal valid message).
But yes you are right, in other cases that are more likely to happen, things would not break for everybody all at once