I'm gonna be majorly pissed if they disable space bar scroll. I use that quite literally constantly while I'm browsing. It is the scrolling method I use 99% of the time.
Backspace to go back, OTOH, I was aware of, but I doubt I ever wanted. Alt-Left is easier to use because it's less modal. Still bothers me in general when devs screw with established defaults.
The most confusing thing is that Youtube and Netflix seem to occasionally toggle pause when you press space; other times Youtube scrolls, and Netflix does nothing (there'd be nowhere to scroll).
I've never managed to consistently replicate it by having had the player in/out of focus already, not/scrolled, etc. odd.
Chrome is lately behaving even more unpredictably when scrolling with the spacebar, because now if you click on either the back or forward buttons it puts a focus rectangle on them and now pressing space will go back/forward again instead of scrolling down like I wanted.
Right hand on mouse, left hand on WASD. The spacebar is always easy to hit your thumb, but you have to move your right hand away the mouse to hit Page Down. Plus it mixes well with Vimperator-like extensions, to the point that I can do 90% of my browsing with one hand.
EDIT: and before anyone points out that if I'm already using extensions I could easily add another, yes and I would if I had to, but it's not ideal, because 1. Extensions crash, fail to load, or aren't allowed by internal browser pages, and 2. You sometimes have to use other peoples' machines and it's annoying when basic functionality is locked away.
All keyboards have a spacebar. Not all keyboards today have dedicated page up/page down keys. So potentially more keystrokes due to the function layer.
Backspace to go back, OTOH, I was aware of, but I doubt I ever wanted. Alt-Left is easier to use because it's less modal. Still bothers me in general when devs screw with established defaults.