First, I think one has to recognize that there are a lot of traps that an unseen servant will not be able to help with, in particular complex or magical ones or anything triggered by weight or force greater than it can produce. Can you even see the parts of the trap the unseen servant needs to interact with from your position and are those things within 60 feet of you? Do you know how the trap works so you can tell it what to do? So this isn't a tool that's going to work in all or perhaps even most cases, depending on the content the DM is presenting. In the abstract, wow, unseen servant is a great trap springer! In reality, it's not the cost-free victory it's made out to be, unless the DM's exploration challenges are about an inch deep.
Well, let's dig into this. The spell specifies that the servant "can perform simple tasks than a human servant can perform". So, what kind of complex trap are you talking about that I need to see the parts of? Most traps have three types of triggers. Pressure Plates, trip wires, some sort of lever or magic.
And any trip wire is easily taken care of. Pressure plates would have to be able to take more than 60 lbs of force, which is a lot. I don't need to know how the trap works to trigger it this way, just like you never needed to know what a trap did to trigger it with a ten foot pole.
Levers? Well, the unseen servant can certainly pull levers. So, if I'm let's say 30 ft back outside of the room and pull a random lever, how many actually dangerous things can happen to me? There are very few things that would be dangerous to the person pulling the lever, that would be dangerous to people who are 30 or more feet away.
But, there is still magic. Well, how is it a magical trap? Does it trigger if a creature gets into the area? Then the Unseen Servant triggers it. They have enough qualities to count as a creature. They don't trigger it? Then if I can see the trap, they can begin messing with it. Maybe they can, or maybe they can't do enough to it to disarm it, but at a certain point of the trap being unable to be seen and unable to be interacted with, there isn't much more that the rogue would have been able to do differently except get caught in the trap.
Next, setting off the trap isn't always desirable either. It may keep the PCs safe (then again, it might not depending on the area it affects), but what else might be negatively impacted by the trap going off that is important to the PCs? What if that pressure plate causes a huge block to drop down from the ceiling, destroying the unseen servant, and blocking further passage? Oops.
Then it is better that it happened to the unseen servant than the rogue. And if the passage is completely cut off and there is nothing we can do... then we leave the dungeon and go looking for something else.
The problem with your example is though... no DM would do that, well, I guess Gygax and some hardcore 1e DMs might, but what you have described is a single point of failure for the entire dungeon. What would you do if you set that trap up, and the rogue rolled a 1? Kill the rogue, and lock the dungeon forever, making the entire journey a waste of time?
You should never put a trap in you aren't willing to have go off, and something that destroys the Unseen servant but doesn't hurt a party member is a win, because that trap going off and killing or harming a party member is far far worse.
And if it is meant to destroy the thing the PCs are there to get... well, first of all, then the trap is usually obviously telegraphed, because the entire goal is to avoid setting off the trap in the first place. And then we still have a formless, flying invisible force that can interact with anything we need to interact with, making the entire endeavor far safer than having the rogue try and reach whatever it is we need to reach.
Finally, the position I have taken is not that spells or the like don't help with the exploration challenges in some cases, only that rarely are they "free." They come with risks and trade-offs just like anything else - same as combat or social interaction options - particularly if the DM has created urgency via countdowns or random encounters based on time or noise. It's here where meaningful choices are provided to the players: Do I cast this unseen servant spell and risk a wandering monster check? Do I set off the trap with the unseen servant and create a bunch of noise? Is setting off the trap actually going to help us here? And so on.
But wandering monsters aren't an exploration challenge. What you are saying, in effect, is "do I solve the exploration challenge, by creating a combat challenge". And, at some point, you run out of monsters. Or, you run out of logical places for the monsters to come from. If we've cleared an entire floor of the dungeon then retreat to a room and lock it to summon the servant... where is this monster coming from? From down the stairs? Okay, good, then we killed it now instead of dealing with it later.
And the opportunity cost of knowing Unseen Servant is far far lower than the opportunity cost of the rogue's life. So, it is certainly worth having.
Which leaves us... countdowns. Which sure, countdowns are great ways to counteract what I'm talking about. But that isn't providing a challenge Unseen Servant and these other spells can't overcome, that is forcing the players to act inefficiently because they don't have the time to be safe. But not only is it nearly impossible to have a countdown for every single exploration, but it again is just tilting away from the exploration pillar and having us deal with a different aspect of the game instead.