This is such a weird argument. Adjust your expectations -- and play five minutes each week? Combat, even in OSR, makes up a significant part of any adventure.
I've been playing since 1979, so this isn't a case of me not knowing how the game was played at the time. Telling one of the players at the table to just sit quietly during most of the game sucked then and sucked now -- and it's a pretty crappy way for designers to treat a player and a crappier way for players to treat a friend. You wouldn't invite someone over to play a board game with you and then, after five minutes, just force them to watch everyone else play for hours.
If I'm playing a low-level mage and I'm outta gas for the day I'm still going to find a way to contribute to the fight somehow, even if it doesn't mean I'm actually doing damage to the foes.
Also, I'm fine with the long-term trade-off where I support them now because at higher levels it'll be them supporting me.
To answer your question more seriously,
@M.T. Black, I never found a satisfying answer for this, other than to have people play multiple characters at a time, so they could still have something to do,
I always allow a player to run two characters at once, mostly so that if one of them dies (a frequent occurrence at low levels) they've still got the other.
Back in the day when we were continually rerunning B2, T1 and L1, most people just stopped playing magic-users in favor of other classes, which IMO was a game design failure.
MU was always intended to be the least-played class anyway. Gygax assumed that 40% of characters would be warriors (F,R,P), 30% Clerics/Druids*, 20% Thieves/Assassins*, and only 10% Mage types. (I don't know where Monks were supposed to fit in)
And over the years I've found there's rise-and-fall trends as to which class(es) generally don't get played. For a while nobody played Clerics. For another long while nobody played Thieves. There's been runs where nobody had a Mage. Warriors, however, are pretty much evergreen.
Now I have made things a bit easier on low-level MUs in that they start with 3 slots a day (so do Clerics) instead of just 1, but that's about it.
The end result is that in grand total my games have been more or less 40-20-20-20 F-C-T-M (well, more like 38-19-19-19 with the remainder being a smattering of Monks and Bards); so if anything Mages have been more frequent than Gygax intended, not less..
* - or the reverse, I can
never remember whether it's 30-Cleric 20-Thief or 30-Thief 20-Cleric.