it's pretty unlikely to fall down now!"
That's another odd piece of reasoning I hear frequently -- I just ran across it this morning in a work of fiction, in reference to a house, but I've also had it said to me personally, regarding other engineered works. Of course, everything has "been standing" until the very day it falls, so this almost predicts nothing will ever fall. And, of course, everything collapses eventually, so it would be more accurate to say, "This is getting closer to the day it will collapse every minute!"
I think the intuition is that, "If it's stood this long, it was built well." Now, it's true that if a bridge has held up for 80 years, that means it was engineered well enough to stand for 80 years! But the jury is entirely out, based on that fact, as to whether it was good enough engineering to keep it up for 80 years and 1 day. You could, of course, inspect the bridge and see if it's still sound, but that is exactly what this old saw is meant to discourage -- in fact, that was the exact situation in which I once heard it used:
"I should get this wall looked at."
"Nah, it's held up this long -- it's not gonna fall now!"