AoMr and AoSg (and AoB, to be technical) all carry the possibility to get the Backpack, although that's already requiring some massive luck (whether it be blaster/jackal, or plasma shotgun(?), or just an appropriate amount of phases) by the time the Wall shows up, which can be as early as floor 7. If we consider the odds of finding a Megasphere in a game that isn't A100, I don't think it would ever be a problem that players would find it too easily. We could create an exception that they can only appear in vaults (which are nasty little rooms, not to mention no explosions) to create a sufficient challenge on the rare occasion they appear.
The only one that doesn't allow for a Backpack is A100 which, in my opinion, really biases the player to wanting Ammochain in order to eliminate ammo conservation problems. I just finished an A100 game without MAc and only got through a couple of tight spots because I was lucky enough to have BOTH the nuclear plasma and nuclear BFG. Cells are already a problem, taking up double the space of bullets, and bullets are used up FAST after a few floors. Rockets become a staple but prevent you from collecting most ammo (same is true of BFG) so you can end up in some nasty, ammoless situations, especially on harder difficulties (except N!...but that's a bit of an exception). The chance of finding a Backpack early enough would be a nice relief: 40% increased capacity is significant, lemme tell ya. Should it end up later...well, it's still plenty possible, I managed without the nuclear gear until the 90s.
(Just as a side-note, I managed the run using neither MAc nor Intuition. It was HNTR and I was bored, what can I say.)