For people that are just starting out, streaming is what majority of the people will go for rather than building their own local catalog. You just pay a monthly fee and you get almost all the music you can have right at your fingertips without the hassle or price of buying out albums, and maintaining a library. These streaming services also allow for playlist so you can make your library within them.
I have Roon and Spotify. I have auditioned Tidal twice and had Sony's Unlimited Music many years ago. I found Spotify to be far above what Tidal had to offer in terms of a larger catalog, curated playlists that actually made sense, having a UI that's much better. I stick with Spotify for offline playback and when travelling. I know other services offer lossless, but I'll take Spotify's MP3 over the lossless offered by other services. If Spotify ever did go lossless, that'd be the only time I'd consider it.
With respect to Roon, there are people that love the integration of those streaming services within Roon. Unfortunately, Spotify will never integrate into Roon, so for myself, Roon is just local music. Since I have a big local library, I don't feel the need to have a streaming service associated with Roon. Also, paying for two streaming services doesn't make sense (to me).
If I was listening only via streaming services, I don't think I'd pay for Roon on top of that just to get a different interface for that streaming service. I'd try to use the streaming service natively. Although, in mine and many other people's cases, we wouldn't go with anything but Spotify, so Roon integration is DOA.
I think people with big libraries or that need a specific feature of Roon (multi zone) will still continue to find Roon useful. Although, I do come across frequent posts of people complaining about classical music integration and organization.
Roon with a streaming service alone only makes sense if you make use of Roon's features and try to make your own library. If you're simply using Roon for basic playback, it's too expensive as just an alternative interface.
There's actually a thread or two on Roon's users of how many people have just local music vs. streaming service, or both, and how many songs from each category. That might be useful to look at as well.