@rebdoniel said:
“on the grounds that this would basically take prestige and usefulness from rebbeim”
That reason sounds very unlikely, given that the work on the ???? ????? ?????? predates the Artscroll shas and is in universal use.
Rabbeim would have no problems producing an interesting shiur, daf hayomi shiurim are increasingly popular despite many using Artscroll gemorahs.
Anyway, ?? ?????? ???? ???? and there’s always more to be said as evidenced by countless compendiums of chiddushei torah al hashas that are produces despite the ???? ????? ?????? existing.
More likely is that such a task is too huge, and they have to limit themselves to what’s possible, as you indicate. The ???? ????? ?????? is far from complete and a much larger work than the Artscroll shas.