Best answer by Ryan S
To be clear, the Sonos One can be used with all Sonos players. It can control what's playing on itself and other rooms with voice, where available. However, Sonos One will only pair with another Sonos One if you are looking to set it up as a stereo pair or as part of a surround setup, it can't bond with a Play:1.
While the Play:1 and Sonos One share a similar appearance and sound, the two are aesthetically and acoustically two different speakers. Stereo pairing the two is not possible through the Sonos app.
This comes down to the hardware in the players themselves. You're going to start seeing more differences between the Play:1 and the Sonos One when it comes to features in the future, as new things are added to the system that might not be compatible with older hardware. We strive to keep products working with core functionality for as long as possible. As we add new features, older products may have more limitations.
You might have heard recently that we're bringing Airplay 2 to Sonos on a selection of players. The Sonos One will be available as an Airplay 2 target, but the Play:1 will not be. The first Sonos players ever, the ZP100, is still out there rocking today, so don't worry about the life of your Play:1s. That one won't be an Airplay 2 target either, but you'd be able to group a Sonos One with a ZP100 and play Airplay 2 to both, just like with a Play:1. The group is different from stereo bonding.
There are some community suggestions for ways to bond a Play:1 with a Sonos One, mentioned in this thread, but they aren't officially recommended. They may work for what you're looking for, but the setup could potentially be unstable.
Thanks again everyone for sharing, and we'll make sure the team knows this is still something you'd like to see happen, but there aren't any plans to share on bonding a Play:1 and Sonos One as a pair.