Why would the same-level stuff wait for the other stuff to die before moving in?
Your assuming the new stuff was just sitting around, waiting for the stuff inside to die, so they could go in. What if you wipe out all the level 5 stuff, then the cave it totally empty for a week. Then, a nice troup of level 15 stuff wanders in through the wilderness, finds the empty cave and desides to stay. That makes perfect sense, and it allows stronger stuff to be inside the cave upon reset.
I tend to get into a lot of arguments about stuff I don't give a damn about..
Bandit's would make sense, as long as they're equipment is kept low even at high levels, seriously if you have glass armor just sell it you'll make more money then hitting up peasants.
Ghosts and Draugr and what have you wouldn't. What are they just floating around from tomb to tomb? No. Why would treasure replenish? The undead don't horde stuff, they may protect stuff, but there not going to pick up an axe from some dumb adventurer and put it in the respawn chest. Why would the ancient undead guard a rare deadra helm or something and use there weakest people before there toughest. "Oh we lost the helm, next time use the grade A guys. for .when that guy who stole the helm comes back in for some reason."
If anything when the tough things die, it's probably more probable for weaker stuff to move in, with all those skeletons dead some bears and rats move in. It shouldn't just be straight up everyone gets tougher, it needs to flow or just stay the same (which would be the easiest for the devs)
or something.