opt-in or explicit tagging of fee account is a bad design IMO.
As pointed out by James O'Beirne in the other email, having an explicit key required means you have to pre-plan.... suppose you're building a vault meant to distribute funds over many years, do you really want a *specific* precommitted key you have to maintain? What happens to your ability to bump should it be compromised (which may be more likely if it's intended to be a hot-wallet function for bumping).
Furthermore, it's quite often the case that someone might do a transaction that pays you that is low fee that you want to bump but they choose to opt-out... then what? It's better that you should always be able to fee bump.