Why is so much done with sliding up/down now instead of buttons?

For an “improved” user experience. Hmmm yeah right...

Not a fan of that either TBH. 
the old apps may have not been perfect but still far more useable that this PoS

Yes, the ‘slide-screens’ are making appearances in quite a few updated iOS Apps - seems to be the new design, in place of tabs.

I’m okay with the new Sonos App, it works fine here with my setup. I prefer the phone App compared to the tablet App, but my ol’ peepers prefer the larger display these days. I would like to see some of the tablet screens/overlays fit better on screen in some areas, (eg. Now playing and queue playlist screens), as shown in the attached.

The App seems to be (better) designed for the smaller phone display.

I opened up my Sonos app to see what you are talking about and the app crashed. repeatedly. Back to SonoPhone it is. ‍♀️