The documentary is about the original Playstation though, and it was Sony's first system so backwards compatibility wasn't really part of it.
And yes you're right that backwards compatibility has certainly helped.
N64 to Gamecube to Wii had no compatibility. Saturn to Dreamcast. Xbox has had compatibility though I thought?
PS2 could play all PS1 games because, but my PS3 could only play PS1 discs since PS1's GPU chip was so cheap to include. They took PS's Emotion Engine out of it to get price down after first gen. It mattered less later because they re-compiled PS2 games to run on the PS3.
I never had a PS4 so I'm not sure if it could do PS3 game discs.
I haven't tried a PS3/PS2/PS1 disc in my PS5 yet.