Another topic that started on the RPGSite forum at about the same time as the level drain debate was multi-classing. The question was "why does the old school hate multi-classing?" which is something I haven't seen in real life. Thus, I don't find the question that interesting. (Although to answer it, perhaps what new school players perceive as "hate for multi-classing" is really distrust of multi-classing schemes that feel like munchkin optimizations.)
But what I think might be a more interesting discussion is: what would be a better restriction on switching classes than ability score minimums? A character can switch to a new class if their score in the prime ability of that class is 16+. I see racial multi-classing as just a special case of switching classes (no minimum score needed, can switch back and forth between two or three classes.) And that appears to be the opinion of the authors of Delving Deeper, since it's an official part of those rules.
However, a lot of people don't like 3d6 in order and opt for other schemes, like 4d6 arrange to taste or point buy. These choices make the 16+ minimum score requirement irrelevant. It seems to me that there ought to be some downside to switching classes, so that class switching isn't the automatic best choice, making single-class character undesirable.
There is one minor downside to class switching, at least the way I run it in OD&D (and the way it works officially in AD&D.) Characters with more than one class use their highest level to determine hit dice, not the total of all their levels. Actually, AD&D phrases it a bit differently, but it's nearly the same result. The rule's effect is that characters with two or more classes have slower advancement.
Another good restriction, not included in any version of the game I'm familiar with, would be: once a character reaches name level in one class, the character can only advance in that class. You can only reach name level in one class, in other words.
A rule that I've hinted at before, but should formalize, is: instead of meeting a minimum score, subtract 8 from the character's effective prime ability score when determining experience adjustments. This means that a character with a 16 in their second class does not get any experience bonus, and even a 15 will result in an experience penalty. Switching back to the original class carries the same penalty.