TCP sequence numbers are not a security mechanism, they are a data ordering mechanism. If you want security, use encryption like IPSec or TLS. This has been the state of things since TCP was invented in 1980. Someone new discovers it all the time.

They are effectively authentication cookies, but the cookies are too short, but at least they turn over frequently and you have to send a few gigabytes of data to guess one.

Almost every internet security mechanism relies on nobody guessing secret numbers, but normally they are sized appropriately.

That's why we had to invent TLS (and now QUIC)