> This makes a lot of sense as it matches the semantics of what we are trying
to achieve: allow the owner of an output (whether an individual or group)
to reduce that output's value to pay a higher fee.

Note, I think you're still struggling with some trust issue that anchor upgrade is at least eliminating for LN, namely the pre-agreement among a group of signers about the effective feerate to use at some unknown time point in the future. If you authorize your counterparty for a broadcast at feerate X, how do you prevent a broadcast at feerate Y, where Y is far under X, thus maliciously burning a lot of your fee-bumping reserve ?

Of course, one mitigation is to make a contribution to a common fee-bumping output reserve proportional to what has been contributed as a funding collateral. Thus disincentivizing misuse of the common fee-bumping reserve in a game-theoretical way. But if you take the example of a LN channel, you're now running into another issue. Off-chain balances might fluctuate in a way that most of the time, your fee-bumping reserve contribution is out-of-proportion with your balance amounts to protect ? And as such enduring some significant timevalue bleeding on your fee-bumping reserve.

Single-party managed fee-bumping reserve doesn't seem to suffer from this drawback ?