If they went all-digital, Sony and Microsoft only need to care about Best Buy sales insofar as Best Buy is motivated to carry console hardware.
IMO, the biggest argument against all-digital is what happened with Xbox One launch. But time has passed, the PC market has swung heavily toward digital, and digital has tons of advantages for platform creators. Digital means no used games stealing sales, no physical storage and distribution, no channel partners taking a cut, cheaper hardware with no/fewer moving parts, etc. And streaming services like PlayStation Now, while not good enough for fast, competitive games, are fine for many other kinds of games.
I don't know if I really think the 9th generation is the one that will go all-digital, but I have a hard time imagining the 10th generation (PS6, etc.) will still support physical media.