Earlier quoted context omitted.
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.
Aside from it just being what you're used to, what's wrong with just having PgUp/PgDn for scrolling?
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.