I'm with you on that one. I've been sandbagged a few times, some of them made really good camp fire stories, but others I simply try to forget and thank the stars I'm still alive. I think what makes trad routes (some of the time) hard to grade, is the fear factor. A high risk move will automatically feel harder, a series of such moves can very suddenly become madness, even though on toprope you'd easily be able to do it. Sometimes the crux of the route is placing the gear. All of this makes it hard to grade, but I agree, every now and then, I come across a route that is just so wildly sandbagged that I honestly can't fathom how it was graded. I've often thought of going through the trouble to suggest a grade change, but have never done it. You are right, for some reason we just don't upgrade trad routes, because it makes us softies. Maybe we should just point out the dangerous ones on the route guides as we encounter them. All I have ever done was to add X's and R's on the
trad hit list where I felt it was needed and off course, the stories. I'm not a grade chaser, but I agree with you, a nasty sandbag can get you into serious trouble! I don't think it is unwise to at least warn people about it.
My list of notable sandbags that I can recall from memory:
Spiderman 17 (Makapan's Gat) - This route is simply insane, I can't even see how strong climbers can do it. I had to aid quite a bit and even then it was horribly hard! My two seconds couldn't even do it on top rope.
Salesman 15 (Tafelberg) - I simply couldn't stay on the steep line of slopers, I had to go right.
There are more, but I didn't think they were out by more than 2 or 3 grades (or maybe I just cleared it off my mind).
Then there's the death routes that's not sandbagged but dangerous:
Last Rites / Coffin (Upper Tonquani) - 1st pitch. I don't think anyone will ever be sandbagged on it, it is too notorious for that, but the RD makes no mention of the mostly unprotectable 1st pitch - that is ironic enough graded as the easiest pitch of the 3.
Shut up and die like an aviator (Boven) - There's just nothing good about it, shit rock shit gear.
Amazing Grace and the Beer Hunter (Hamerkop) - Amazing routes, but hardly any gear.
Solitaire (Spout) - I have to put it on the list, because the gear is few, far between and mostly useless. So maybe not a death route, but definitely a ARRRR rated route with a capital R in bold

It is also maybe a little sand bagged, but it's hard to say since I didn't exactly top out.
I see Brian mentioned in his article about under-graded routes in lower grades. I've seen the same thing with trad routes. Labyrinth in Cedarberg kloof for instance is graded at 10. If you are too short to get into the chimney on the second pitch, you're going to struggle.
I know I probably went off topic a little with the dangerous routes, but I guess it comes down to the same thing (and reasons we don't change it).