Even if the newest firmware works with the old app for now, future firmware may not.  I suspect there will be more firmware updates to coincide with app updates and Sonos themselves in their AMA have hinted future firmware may not be compatible with the old app.

Regarding the idea of a separate app, the problem with this approach is that the functionality between these apps would diverge over time, leaving a large group of users with an out of date and potentially not fully supported experience.

Thanks for your reply. I’m going to hold off on updating (if it’s not forced on me) until Sonos returns a function I do use, such as queue management.

I stupidly updated on my phone but other devices haven't and they work fine. 
However there is a firmware update already released that breaks the old app. 
you'll want to go into your system settings in Sonos and turn off auto update. 

I've got my Android phone app rolled back and the iPad app on the newest version.

Android won't let me unpair speakers for example, asks for an app update.

iPad doesn't let me tune Trueplay, the app softlocks.

No proper solution in sight.

I’ve long had automatic updates turned off, not only on Sonos, but on my Apple devices as a whole. Until last summer, the only Internet service I could get at home was DSL, which was so slow that if an update was happening, I couldn’t use any other devices at the same time. So I wanted to be able to schedule the updates myself at a good time when they wouldn’t interfere with other things I needed to do.

I now have Home 5G service, which is more than 30 times faster than the DSL was, but I guess I still don’t like update surprises! However, I should have waited to update my one device until I learned more about the new Sonos app changes. That’s my own fault, mostly, but I did rely on the email Sonos sent out saying all the same functions would be available in the new app.

I did get a Sonos app popup on a non-updated device that my speakers wouldn’t work until I did the systems update. However, they’re still working anyway, even though I chose not to update, so I will continue to hold out as long as I can. Thank you both for letting me know how the newest update has affected your devices.