Alright let's talk about posting guards and stealth/perception resolution.
Presumably I post guards in an area to defend it from theft or intrusion, so what good are they if they get snuck up in and murdered in round 1?
Fair question. But it boils down to how you resolve that ambush.
The players intend to sneak up on and kill the guards. The guards are "on alert" for just this circumstance. You're the DM, how do you resolve the outcome?
The rules say to roll stealth against passive perception (add 5 if the creature has advantage). Good deal.
But is that the end of it? 1st, I posted guards that I want to be alert who should be actively looking for danger. So as DM, I'm at least putting advantage in. 2nd, I would like to minimize the opportunity for someone to hide (not eliminate, bc I want to have the stealth option open for players, just risky). And 3rd, I need an x-factor like a guard dog or something with a sense of smell that will signal or alert the guards and make the ambush impossible.
If I have these components together, guards retain their usefulness. Players are confronted with a meaningful choice about their approach to the thing I'm guarding, and a question of priorities (deal with the dog 1st, ignore the dog, wait it out for an opportune time, just rush in, some combo?)
Maybe we're insufficiently looking at in-game circumstances when we're resolving perception and stealth. Remember that the roll is the last damn thing you do before game circumstances change, before there's surprise or no surprise at all. And the best part of it isn't whether or not the players ambush the guards, it's the play that leads up to that roll. It's the riskiness and tension that either pays off gloriously or fails spectacularly that's the fun part and where (IMO, of course) our focus belongs.
Side note - I've noticed a lot of this over some years, and recently. It's a tendency to look at rules and conclude that they produce undesirable outcomes. Say what you will about various rules, that's all good and certainly there's room for improvement, but these things exist to facilitate in-game action and resolve ambiguities - not to create action or take the place of a narrative. In other words, it's important for me to remind myself that this game (and lots of others) devolves into boring unplayable gibberish when we (I, really) treat it like "rules first, throw dice and numbers at our obstacles until there aren't any more numbers or dice or obstacles left."
-Brad