The answer is a pretty easy yes, and one could even more easily say they're the ONLY client on PC really innovating still.
GoG is the closest to competition they have with DRM-free as a unique selling point, but the rest of the clients out there are just repositories for certain publishers to sell their own games so they can avoid paying Valve a 30% sale cut. I've seen little to no innovation from any of these clients, just partially copying features Steam already has.
Despite how numerous other clients are from console-focused publishers, Steam still the only one that has a Big Picture Mode meant for playing on the TV, and can be ENTIRELY controlled with a gamepad to launch titles that are marked with controller support. They created their own gamepad with the most robust and customizable input API I've seen, and then made that API compatible to all the other major third-party controllers. Then you have Early Access, user guides to games, one-click subscribe Steam Workshop mods, Family Sharing, easily superior sorting options and wishlist features, baked in discussion forums per game, selling Trading Cards if you want free money, Steam Curators, Steam developer pages, etc. Barely any of these features have even been copied in other clients.
I would agree that Valve needs to overhaul and clean-up their default UI, but their beta update in the last month shows they're already starting with the complete revamp of the friends list. Steam Skin makers already see that a big change is coming in that area.
See little reason to use other clients unless I have to for access to certain games, which I don't mind...but every other client is so far behind Steam in so many areas it's not even funny.