Lightning Network – Why does a single expired HTLC with an unresponsive peer automatically kill the entire channel?

As I understand it: If a single HTLC expires on a payment channel with an unresponsive partner, the entire payment channel will be forced to close (action A). This was discussed, for example, in this question. Adding commitments to Bitcoin allows participants to settle only expired HTLC on-chain without having to close the entire channel (Action B).

First of all, is my understanding correct? So why is it impossible to implement behavior B using pre-signed transactions today?

I would expect the parties to make multiple pre-signed transactions to settle an on-chain channel, i.e. one transaction that closes the entire channel (as it does now) and another transaction that leaves the channel open (i.e. 2/2 multi-signature output) on the HTLC blockchain. Settlement. Is it technically difficult or theoretically impossible without a covenant?