Replace Russound keypads with Sonos? I have about a dozen Russound MDK-C6's in different rooms.

Sonos doesn’t make keypad controllers, so I’m not exactly sure what you’re trying to accomplish.  From what I understand, these controllers work with Russound’s mca amplifier.  Are you wanting to replace those with Sonos amps?  You could do that, and you would need a Sonos amp for each room/zone you currently have. That would make your controllers useless, but you could control everything through the Sonos app.   Alternatively, you could keep the Russound amp and use multiple Sonos ports for each audio source you want to use.  You would use the Sonos amp and your wall controllers to control the system.

If you get rid of all your existing Russoud amps and controllers, using Sonos amps, then you could possibly replace the keypad controllers with Apple or Android tablets, using the Sonos app for control.  There are also some third party wall controllers, not made by Sonos, that can control Sonos as well as other smart home devices.  I don’t know that I go that route, as you could simply remove the Russound controllers and use a blank face place, or pay someone would patch the holes.