Thinking about Gygax's remarks in Book 1 that wizards are strong at high levels but weak at low levels to make up for that strength.
Reserve XP in ACKS gives players a way around this - level with a fighter, invest heavily in reserve XP, and then if you die you can bring in a mage at a level where they don't have it so rough.
That I have been playing Grim Dawn recently may have also contributed to this thought - it's very normal there to pick skills that make leveling up easy, and then to swap them all out at high levels. I don't love it there either.
One solution is to just remove reserve XP. But I like having a rule that provides some tangible benefit to partying hard (and doesn't require wrangling the Cost of Living table). Some of my players do complain that reserve XP is sort of a trap option; I think I might do a more thorough analysis of it at some point.
Another solution might be to make reserve XP pools class-specific (or something like function-specific; fighter and vaultguard share the same pool, cleric and craftpriest share the same pool, explorer and assassin (maybe, I feel like assassin scales better while explorer starts strong), etc). So you can't level as a class that starts strong and then convert your reserve XP into a class that scales better once you start stalling out.
I don't think this is totally crazy from the perspective of "XP is score, reserve XP is a way to preserve some score across death". Score is already class-specific; why should preserved score not be?
A few possible reasons it might not be:
1) There are already other easy ways to bring in MUs who are past or almost past the Threshold of Deep Suck, namely hiring 3rd-4th level henchmen. On the other hand, hiring henchmen who are in the 6th-8th range is hard; bringing in a character on reserve XP at those levels also requires commitment to the strategy, but seems pretty doable across multiple lifetimes of normal play. I'm not so worried about henchmen who you level up, since you're already paying a bunch of XP and gold for them and having to keep them alive.
2) Tying reserve XP to a particular class works at cross-purposes to prime requisites, which (I think) are supposed to encourage players to play to the stats that they roll and try a wide variety of classes. If your reserve XP is all fighter XP and you roll stats that are good for cleric, that reserve XP encourages you to shoehorn those stats into fighter and to play your usual type.
3) Given changes to some of the more powerful spells, is the balance between ACKS' MU and ACKS' fighter such that MUs are the most powerful at high levels? Is this change senseless as a result? I'm not sure. But there are a set of classes in ACKS which are regarded as super-scalers by the discord. I forget whether MU is among them (the bulk of them were in HFH I think). And there are certainly other classes that have an easy time at low levels but cap out early. So even if MU isn't specifically our example for scaling, there's still a somewhat exploitable mismatch here.
4) Extra bookkeeping, having to track reserve XP for a bunch of classes, especially if you rule that reserve XP is a floor and not spent to bring in new characters.