Why knowing how much time is left in a movie should not be allowed

So today I was watching "Bride of Frankenstein".
The old one.
It was made in nineteen thirty . . . . something.
Anyhoo, I knew how long the movie was.
And it seemed to be taking FOREVER to get to the point: they only made the bride in the last 20 minutes,
I call that a rip-off.
But!
I was just thinking . . . which can be dangerous, but whatever.
I could sort of tell how the movie was going to end.
Why?
Because you can't do something complex in 20 minutes.
Unless it's "Inception", which it most certainly wasn't.
I could tell it was going to be a sort of sad ending, because how are you going to make, marry, and maroon (alliteration, sorry, I couldn't help it) a she-monster in 20 minutes? Especially when you spend about half that time going on and on about the storm?
(On that subject: why can't monsters ever be made on nice, sunny days? Maybe the atmosphere they were 'born' into had something to do with their later behavior?)
I digress.
I guess what I'm trying to get at is this: you shouldn't know how much time is left in a movie, because too many things are no longer mysterious or surprising because you can figure out what they do with X amount of time.
Unless it's "Inception".
Duh.