Roku has tie up with TV manufacturers like TCL too. That's where a lot of their growth is coming in.
Personally, I have switched to Fire-Stick and Chromecast with Google TV from Roku because the Roku interface hasn't evolved in the past few years and they are also pushing ads and their own channels now.
I recently managed to get my hands on a non-smart TV and bought a roku stand alone device for it. Mostly because the interface is simple and has been the same for years.
I prefer the moving fast and breaking things to stay away from our limited TV time.
Personally, I have switched to Fire-Stick and Chromecast with Google TV from Roku because the Roku interface hasn't evolved in the past few years and they are also pushing ads and their own channels now.