But I don't entirely agree with your last paragraph. As a 4th level wizard in 1st ed AD&D I have 3 first and 2 second level spells. At 5th level I get a new 3rd level spell. So time to rest and memorise all spells at 4th level is 4 hours + 7*15 minutes is a bit less than 6 hours. I think that's less than 3E, which I believe for arcane casters requires 8 hours (though I think RC is right that it's less harsh as far as interruption is concerned). At 5th level it's 6 hours + 10*15 minutes which is eight-and-a-half hours. That's not very much more than 3E. I can't remember the spell table past this level - but even supposing that 6th level adds both a 2nd and a 3rd level spell, that's still a little less than 10 hours, which isn't hugely more onerous than 3E.
And once we get above 6th level, we're getting into the zone where 1st ed MUs are pretty strong - multiple fireballs or lightning bolts, stinking clouds or webs, and multi-target magic missiles, and often a wand or two to use in combat as well. Plus the utility options available. In my experience 7th+ level AD&D MUs were pretty potent characters regardless of the % chance to learn mechanics.
The rules for AD&D state:
As a rule of thumb, allow 15 minutes of game time for memorization of one spell level, i.e. a 1st level spell or half of a 2nd level spell. Such activity requires a mind rested by a good sleep and nourished by the body.
I used to play the Gold Box TSR computer games, including Pool of Radiance. In my experience, spell memorization was a big issue, as this computer game brought to life in a way more prominently than a human-mediated game. Taking into account the prevalence of wandering monsters, which could occur hourly, it was difficult to find the 2-3 hours needed to rememorize. If you were lucky, you might be able to replenish. However, after a couple of memorization periods, your characters would tire, and need to sleep again.
The way this usually played out was that, generally, I would head out with a wide selection of spells, go as far as I could before they were exhausted, then try to head back to a "safe" location (i.e. low wandering monster area, like the bottom of a well, or one or two completely closed rooms, or the inn in town). OTOH, when I knew I was heading into trouble, I tended to load my magic-users up with all the stinking clouds and fireballs I could, arm each cleric with one hold person and all the cure spells I could carry, and attack my opponents fresh. If my cleric were rendered unconscious, I would sometimes have to resort to cheating on my save games, as it was sometimes necessary to rest for days for an unconscious cleric to revive, and then be able to heal themselves. Ultimately, I tend to fall back on a two cleric, one magic-user, one elf fighter/magic-user as the mainstays of my party.
3e is a picnic by comparison. All you need is eight hours of sleep, and if you get interrupted, you can make up for some lost time. Memorization doesn't take all that long. OTOH, if you run out of spells completely, you are kind of screwed, but you really aren't any worse off than your AD&D counterparts who had to try to squeeze in an extra period of memorization in a day. My PCs always just use staggered shifts (generously configured for the benefit of casters) and camouflaged their position. Sleeping in "dungeons" was almost unheard of, though a couple of times they did construct a small improvised vault inside of a "clear" dungeon.
Ultimately, clerics and magic-users in AD&D were very interested in acquiring wands and staves. By comparison, 3e clerics and wizards have a fair amount of juice, and can
make items with fairly little fuss at low levels.
