A first level M-U gets one god damn spell per motherfucking day. They can't even create magic scrolls to store spells until level 7 (if you use general 1e wisdom). In B/X, a M-U must be level 9 to create a scroll or magic item.
Let's drive home this point a little bit more. A M-U cannot even read a magic scroll without casting the spell Read Magic. Which means, as a first level caster, if you don't take Read Magic as your one and only spell, then you can't read scrolls.
"I'm glad I didn't choose Magic Missile instead of Read Magic. I was able to cast Read Magic so I could read that Magic Missile scroll and save the party from that one orc!"
The pushback is obvious and widely-screamed: If you can't think of anything else to do other than cast spells then you suck at this game.
That's fine. Except it's a fantasy game and I came on my Friday night with but one dream: to be a motherfuckin' wizard. And here you got me casting Read Magic and hiding behind the dwarf.
I beseech you, my OSR friends: don't visit this hell upon players. Unless you know for a fact that the person who wants to be a wizard both understands these limitations and LIKES IT, you gotta house rule that shit. And there's a lot of fine, fine house rules out there you can use to fix this broken-ass setup. For example, Magic-Users in your game can...
• Read any scroll.
• Create scrolls and magic items, per the Magical Research rules, starting at first level.
• Cast 3 additional first level spells per day in addition to what's on the spell slot table. Call it baseline training.
• Use any weapon they care to pick up, but if it's not allowed by the rules-as-written, then they get a -3 to hit. No biggie.
• Detect the presence of magic in a device or an area, perhaps by making a simple Intelligence check. You don't have to tell them what the magic is, just that they "get a vibe" off it.
• Possess one trivial magic item at character creation. It's their signature thing, like a hat that returns to them or an unseen hand that carries their sack. I promise you it will not break your fragile dungeon ecosystem.
Ok, this is all just a little bit tongue-in-cheek. But it's also true. If you can't save the Thief class at least pull the Magic-User up from the darkness.