Hah, you say that, but 80% of all games end on Tower:1 and Tower:2
Granted, once you get the hang of things, that tends to stop happening, but even with the various features designed to make them a bit easy, there are still plenty of ways to run into trouble. Malingees, anything involving Raicho, Red Caps if you aren't careful of their limited ranged abilities, etc.
Raicho.. heh... I think it was before your time, but I had to nerf those guys in particular 3 or 4 times to finally find a configuration wimpy enough for them to stay a Tower:2. Even now they're probably the most dangerous thing in the first two floors generally speaking.
Anyway, out of depth encounters tend not to be a big deal in Demon... you often won't even notice when they happen unless it causes a unique to spawn a floor or two early, or causes a major mechanic to show up before it should. Both of these cases are why the "no out of depth spawn" rule was extended from the original just Tower:1 to both Tower:1 and Tower:2: Having West show up on Tower:2 was ridiculous, and Ukobach packs were even worse, especially if either ended up being your first encounter on Tower:2. One of my many criticisms of DCSS is that it's a little too willing/okay with the idea of completely 8-balling you on the first several floors, so it's something I'm trying to avoid in Demon.
But, other than those types of cases, all an out of depth spawn usually means is you'll be fighting a group that has one or two levels on you, and may include some monsters you haven't seen before if you haven't gotten past your current floor in many games. You might need to be extra careful and/or use some items, but I would not expect most cases of this to require a total retreat unless you're completely out of luck and you get an out of depth pack that happens to be the sort of demons that your current team/build is weak against (say, an Angel pack if you're running with lots of undead and demons).