But it doesn't matter if it's reasonable if taking reasonable action is a response to something they really want to do, how often do players go "Gosh, that's ok that we really got boned and died last adventure, because we totally deserved it for thinking we were being cautious and got canned for it."
Reasonable actions on the part of the GM aren't what players are about, they're usually about providing an interesting and enjoyable framework for them to demonstrate how cool and clever they are. If players lock every door in the room, tape the windows shut, cast the Alarm spell and hole up in a magical hut at night, it doesn't matter if it could be construed as reasonable to insert a villain who thwarts each of those precautions - unless you're really careful about it you're just going to come across as one of those sullen, petulant GMs who gets all pouty when players start trying to limit their risk. Especially the bits in the post about "having a talk with them" threatening them with stopping the game entirely for "messing up the game." What's that? They're messing your game up? How...fragile. I think that once you have an intervention asking them to stop doing something, because what they're doing is making it harder to kill them, you probably should just quit the game anyways and write a novel where you can dictate the amount of risk characters take with more precision.
Look, I'm not saying that verisimilitude is a bad thing. I'm just saying that it should probably be non-primary to the nature of your players. Find a way to point out how much time they're expending without slapping them across the face with it, but if they're cool with that then I think you just have to accept that they're cautious. If they're having fun with the static dungeon though, and their weird kick butt and nap routine, I don't know if it's worth doing to threaten that enjoyment for the sake of realism or your plotline, or anything.
If you've REALLY got a problem with it, I might simply ask them why they're doing it. Not "this bothers me, and I want you to stop," but why is this the way you do things? Maybe the balanced encounters don't seem so balanced, and maybe they've even got a better understanding of the mechanics and dynamics of their characters than you do. It wouldn't be the first time a GM overlooked some things about a character sheet because he or she was always busy in combat running the monsters. But I'd be prepared to simply nod my head and let it slide, because otherwise it seems like you're just looking for an excuse to end the game by making it less about the players fun and more about your own.
Most players I've had over the years aren't uncommunicative, if you ask they'll tell you in detail what they're enjoying and why your game sucks. If they're not telling you anything at all though, you're probably not doing anything wrong. Ask them how they'd want you to improve it...Heck, it might be that they're clinging to downtime hoping that there will be more roleplaying opportunities, in which the case the problem isn't the downtime itself but the lack of utilization and glossing over of the downtime.