Obviously this is true of any encrypted chat system. Anyone could be recording your encrypted network traffic as we speak, your conversations will be busted wide open! It's nothing to do with Matrix.
If you're talking about the fact that Matrix stores your encrypted conversation history by default on the server: a) you can set history retention per room, b) if we saw a vulnerability in the encryption we could always go and re-encrypt the history (although it's a bit of a lost cause by that point :)
> On the other hand, if encrypted data is shared publicly on DHT, a replicated database, a blockchain or similar, in year 2050 your nephews will see grandpa's entire message history.
No, Matrix isn’t a public database or a blockchain. The conversations only are only replicated between the servers whose users participate in that given conversation.
You might as well be panicking about PGP archives getting compromised, or backups of devices with Signal history on them, etc.
> You might as well be panicking about PGP archives getting compromised
I don't have a strong opinion on that, but this doesn't sound like an unreasonable thing to worry about. Maybe 20 years ago it seemed like some sci-fi stuff and nobody really thought about it. But then, let's be honest, nobody really thought about e2e encryption back then either. Now this stuff may be not truly an impending disaster yet, but, actually, not so far from it either. Not a big "no-no" yet, but definitely something to be thinking about. And, honestly, I do actually feel uneasy about trusting any secrets with 50+ years expiration date to some integer factorization crypto. Maybe there won't be any practical possibilities to break that in the next 50 years, maybe there will, who knows: but it simply isn't so outrageous of an idea anymore to bet against it. And that's kind of the purpose of cryptography: exaggerating even slightly outrageous possibilities.
If you're talking about the fact that Matrix stores your encrypted conversation history by default on the server: a) you can set history retention per room, b) if we saw a vulnerability in the encryption we could always go and re-encrypt the history (although it's a bit of a lost cause by that point :)