Ok. I think, I might have misunderstood your problem due to the dramatic title and OP.
So, you have those speakers working with the new app, but they don’t show up when you try to add them.
You shouldn’t worry about the Classic app going away. It will not going in the near future. So many thing are still missing from the new app.
As it was advised post you issue in this topic.
Otherwise, there shouldn’t be any difference to add devices from Classic or the new app.
And just for your information, what you were referring in the title that is a bit different situation and it was a bold move by Sonos. But these speakers are not getting bricked by a firmware update, they were designed to work with the Multiroom App, and the app is still supported. (Last update at the end of February 2020.)
Otherwise some comments indicate that network discovery might a bit take long.