Disagree.

I like the new behaviour

Yes me too, it does now feel as if you are walking around in a dangerous world and not some deserted place. It has also helped to fix the slow character levelling issue. Background corruption has been tremendously slowed to accommodate this change, so there is no more a need(well until #430 is gone :p) to rush through deeper levels. Instead you get a chance to play and not just walk.

As I've said elsewhere, the only two levels I'd suggest a reduction in monster count is DH:1 and 2. To lure the player into there.

I agree with the OP, I think the previous amount of monsters was OK. What adds to the fun is dungeon features that are packed with monsters (vaults, tension rooms, threat rooms, etc.) I'd rather have more of those than more monsters packed in the standard rooms and corridors.

It is inconsistent. It is probably a bug - initial spawn on levels is 40-50 monsters, cleared levels spawns barely 20, and usually even less then that, and i dont remember any RFE about changing that.

I don't know where the actual numbers should be - I haven't played pre21. But conceptually, I like the idea that the deep levels start with higher counts than their replacement count. It provides several things:
1) Gameplay: You can start to feel like you've "cleared" a level - or at least made it partly safer - even if it doesn't have tension rooms, etc, because the monster count will decrease.
2) Gameplay: It makes ascending back through previous levels faster, even without on-demand teleport, since there are fewer monsters than there were the first time.
3) Fiction: The lower levels have had time to build up monsters (higher levels always seem to have adventurers killing them), so they're at higher counts - but they drop over time.

It's been adjusted to pre-p21 levels. See http://www.adom.de/forums/project.php?issueid=2895