Even with Streaming and Subscription services I think there's still going to be a relevant market, specially on PC, for people that actually want to own games. And Steam is still the go-to place for a vast majority of those people.
Also, looking at other markets like movies, unless there's a big shift in game budgets and scope I don't think the day one $60 (or $70, or whatever) experience is going anywhere. With current budgets and times, spending $80+ million and 3,4 or 5 years on a big AAA project to release it on a $10 subscription is unsustainable on a large scale, same reason why Avengers Endgame or even Mulan don't release day 1 on Disney + and why the average blockbuster doesn't go straight to streaming but rather starts at theaters, then comes out on Demand and Physical several months later and only then starts showing up on Streaming services.
I think that's the way it will eventually go. Big release for $60-70 and then several months later it comes to streaming and subscription services. In such a scenario I think Steam is still going to be big.
If we somehow get to the point where every big AAA game releases day 1 on a $10 subscription service I guess that's cool for us and bad for Steam. I just don't see it happening.