I tried to get my mother (in her 80s but, for all that, pretty tech-friendly) Netflix for her birthday, which with her 8-year-old not-Smart flatscreen would have meant a Roku box. After I explained it (for the 5th time in my life, I think), she finally announced "I don't want that stuff on my TV. It's too much." Calling it "just a computer" would not have helped. In this case, I think if it didn't require a change of HDMI input when using it she might have gone for it.
Also, even with the Chromecast, you have to control it from your mobile device or computer, and now for some people that's an entire different category of complexity from using a cable remote.