I suppose there's a risk that Steam becomes the defacto way to ship software for Linux which would in effect just assimilate it into being a closed platform.
OTOH that seems unlikely, hopefully it will be more like OSX with a healthy commercial ecosystem and an open source system living side by side on the same OS.
The problem is, that if it aspires to be a generic channel for distributing software besides games, the DRM side if it is bad enough to be concerned, even if it won't replace other open channels.
I'm not sure if it is bad in and of itself. As long as you are free to not install it and stick with your normal apt repos to get your DRM free open source stuff.
I guess it could become a disadvantage if an influx of good commercial software means that effort on open source stuff is diminished because there is less perceived need.
OTOH that seems unlikely, hopefully it will be more like OSX with a healthy commercial ecosystem and an open source system living side by side on the same OS.