As I said quite a few times before: We want to weaken traps so that it is possible for enemies to get by them. Not weaken them so that the enemy will get by them.
But the thing is, right now enemies have no mechanisms for getting past
anything.
This should wait until the siege arc, overall. We still have no idea what sort of AI or strategies enemies will be applying for their siege in an overarching sense, so it's hard to say anything concrete about the details of the traps they will be up against.
With that said, though, I've got one idea: Make traps wear out a bit faster, and require maintenance. If an entire army rushes through your weapon trap (and your dwarves can't get to it for maintenance), it eventually wears out to the point where it stops working until it's repaired. Simple and logical... except for the part where all your engineers suddenly rush into the oncoming army to repair the trap that they set off, of course... which is not so good. Hmm.
Yeah, giving enemies alertness checks is a logical idea, too. This could also eventually be used for secret doors, and would eventually synergize well with adventurer mode (anyone who ever explored a trapped fortress with an adventurer knows how much it sucks to have no way to detect or fight back against traps.) Enemies could have scouts trained for searching and spotting traps... maybe they could use paint or ink to mark traps they discovered (which your dwarves could clean off.) This has the advantage of being much more clear to the player than having them silently go back and report stuff. A marked trap would be easier for enemies to dodge even if they step into it, and naturally they'd avoid stepping on it if they could.
There's still one basic problem, though. Enemies find a trap. They disarm/mark/whatever it. Now, we
don't want to force the player to replace every single trap by hand; I think we can all agree on that (using boredom as a game mechanic sucks.) But if we replace it with an 'active' task to repair/rebuild/rearm/re-whatever, well... we all know what that's going to lead to, don't we? A billion goblins pour in, the first trap gets set off or disarmed or whatever, and instantly every dwarf in your fortress rushes into the oncoming horde to fix it.
Not sure what to do about that. Perhaps there needs to be a designation-style area command to 'pause job' and 'resume job' -- intended primarily to mass-order the reactivation of traps when the coast is clear (and then they'd start out paused.)