I actually think that your statement "...but this is the first time I've really needed AirPlay since installing my Sonos..." is telling.
While I am absolutely not dismissing that having AirPlay built into Sonos would be a welcomed benefit, once you become accustomed to the Sonos ecosystem you may not really miss AirPlay too much.
I also see a lot of complaints from people that the Sonos Play:1 and Play:3 speakers lack an Aux input option. Again, likely something that a non-user might see as a negative, but once in place you likely won't miss it much if at all.
I agree - I've had the Play:5 for over 2 years and the Airport Express for even longer but have never found the need or desire to connect the two together until now
I'm sure that once Apple Music is supported natively on Sonos it'll be disconnected again