There are very few “good” mobile devices for audio. The one that comes to mind is the LG V series with it’s “quad dac” architecture. Otherwise you need to plug a DAC into the mobile device to get the best sound quality.
Because of the way Roon builds its mobile applications it does not take advantage of any hardware specific features of any specific device and always runs the audio via the OSs mixer. So, even my example of the LG V series phone,s it cannot take advantage of the pure capabilities of the quad dac and is limited to the default OS mixer bit depth and resolution (16/48, 24/48? whatever it is these days).
Could Roon change the way they build their apps to provide better hardware support on mobile? Yes absolutely, and this would then provide the same bit-depth / resolution as Tidal, Qobuz, etc. However, that still isn’t as good as connecting a DAC to your mobile device. So, it’s kind of an odd resource investment decision. If the user is serious about sound quality then you expect that user already has a DAC and problem solved. If the user is not so concerned about sound quality then its expected they are good with the OS mixer so why spend the resources and additional testing to spin specific builds of the software?
No one is going to “stop buying streamers and just use a tablet”. That solution sounds terrible (literally, it sounds bad). It might be convenient but it, truly, sounds terrible compared to any purpose built streamer. In fact, I’d even say a cheap Raspberry Pi on battery power as streamer can sound better than a tablet. I’ve not done any blind testing though.