You can readily prove something is in the TXO or STXO set using the actual blockchain, and the proofs will be nice and compact because even light nodes are expected to already have all the historical headers.

What you can't do with MMRs or the blockchain is make a compact proof that something is still in the utxo set, which is the whole point of utxo commitments.

It's totally reasonable for full nodes to independently update and recalculate the utxo set as part of their validation process. The same can't be done for a balanced version of the txo set because it's too big. Relying on proofs as a crutch for using the full txo set would badly exacerbate the already extant problem of miners doing spv mining, and increase the bandwidth a full validating node had to use by a multiple.

This whole conversation is badly sidetracked. If people have comments on my merkle set I'd like to engage further with them, but mmrs need to be argued independently on their own merits before being used as a counterpoint to utxo commitments.