A New Take on Demi-Human Level Limits

This is an idea I've had before, but a quick search of my blog makes me think I've never posted about it before.

So we all know that AD&D had some pretty severe level limits on demi-humans, which Unearthed Arcana and then 2E relaxed. Classic D&D is a bit more generous than AD&D 1E, but you're limited to the race-as-class system. 3E got rid of them altogether and they've remained gone through 5E.

You're probably thinking now, "Thank you Captain Obvious."

I still haven't reverted my Treasures, Serpents, and Ruins house rules to race-as-class (which I'm still considering, but less strongly right now). So my races have limited options for what class they can take, and level limits a la AD&D.

But there's this idea that keeps floating around in my head: What if the level limits only apply to multiclassed characters? 

So TSR classes go up to level 15. Humans have no limits in any classes. Demi-humans do. And unlike AD&D, the highest any demi-human is allowed to go (with the right race/class combination) is level 12. This is to honor the fact that the BX/BECMI Dwarf class maxes out at 12th level. AD&D, as you probably know, allowed most demi-humans unlimited advancement as Thieves.

Well, my idea above would be to allow single-classed demi-humans to reach pinnacles of  power just like a human. It's only when they multi-class that they need to worry about the level limits.

Of course, then some people will ask, why play a human then, if they can't multiclass, and you could get the demi-human abilities along with unlimited advancement as a single-class character?

Well, I do give humans two advantages: dual classing, and Survivability which allows them to roll for hit points with advantage (roll twice, take the higher number). So far, the hit point thing has been a key selling point for the race.

Anyway, it's just an idea that's been rolling around in my head. Not sure if I will implement it or not. We'll see.