The thing is, we likely can’t make unlimited mules in D2:R. There will be character creation limits I believe (and we can’t make unlimited accounts without paying for a new license for each one, unlike the old game where you had a cd-key but could make unlimited accounts).
Shared stash tabs do not make up for the ability to have unlimited mules. Our overall storage space will be substantially smaller compared to the original game with infinite mules.
Making gems and runes stack is at least one compromise. Or having a PoE currency tab style shared stash added just for gems and runes and have them only stack in that tab.