One of the issues with the longer working day is spellcasters tend to end up bored - unable to use spells either because they already have used them all, or because they fear needing to in the future. Was this not still an issue in Basic D&D? Or when Eero says 1 spellcaster per 5 fighters, was the spellcaster also playing a fighter, and thus always guaranteed a slice of the action?
I think the question belies a fundamental misunderstanding of old school play.
Imagine you had a character that had no special abilities at all. Just a set of 3d6 x 6, 1d6 hit points, and normal equipment and weapons and armor.
Could that be fun?
You go into a dungeon, explore, avoid monsters as much as possible, fight when you have to, loot and steal, avoid and bypass traps, and so on. If you run into a problem that you need special skills to deal with--like picking a lock--you pay money for a hireling to come take care of it for you.
You'll probably die. Some of you won't, though. A few will level up and get 1d6 more hit points, increasing the chance that they'll survive more adventures.
Now add one special ability to that character: turning undead, better attack capability, more hit points, maybe one spell.
Modern players tend to look at a character in the opposite way. What explicit abilities are on my sheet? That's what I can do. If my character can only cast one spell a day, I'm a one-shot gun, and then I'm useless. No! You can do everything a normal person can do.