There was a time when bookstore shelves were not dominated by cinder block-sized fantasy epics. Back in the 1960s a 150-page sci-fi paperback was commonplace. Or, so I understand from my expeditions through used book stores. I’m afraid I have no memories from the 60s.
I see nothing wrong with short novels. Not every meal needs to be a seven-course feast. A salad and a half-sandwich lunch is sometimes precisely what you need. At other times you just want to go through the drive-through and get something fast and tasty, but not particularly nutritious.