There are applications that would really not perform well under a whole chain gets locked situation, e.g. anything that requires collateral ratios, things protected by timelocks, bridges.

You can build in things that limit velocity of transactions, send limits etc in the smart contract it's self, no need for it to be at the chain layer, it's just that many of the largest protocols haven't done that (also you either enable some form of DoS or you are vulnerable to sybil attacks)