At some point, Steam's popularity has to bottom out. By virtue of the universe, eventually the service has to peak, but it seems we are some way off of that as the service is as popular as ever.
As pointed out on Steam tracking site SteamDB (via PCGamesN), on Saturday the service hit an all-time high for concurrent players. At peak times, 17,683,804 users were logged in. Now, this is just logged in players, and the Black Friday/Autumn sale is probably a big part of this, but on top of that, 6,831,617 were also playing games. That is a lot for any game platform.
The future is bright for Steam and it has seemed bright for going on a decade now. PC gaming is booming right now and the impact Steam has had on the market can't be overstated. Going forward, it will only grow as Steam grows and vice versa. This is good news.