It's both backward and forward :) A lot of programs are dynamically linked; an older O/S may not support some newer librariers, and a newer O/S may not support

It's both backward and forward :) A lot of programs are dynamically linked; an older O/S may not support some newer librariers, and a newer O/S may not support older ones. While static linking solves this, a hypothetical distribution that is fully statically linked, would take a huge amount space (and memory) - definitely one order of magnitude more (potentially more - a statically linked hello world in go is something like two orders of magnitude larged than the dynamically linked one, although this is an edge case).