Can you not just setup the echo to output through your Sonos system?
Use the echo as a mic, and the legacy Sonos speakers as speakers?
I've just done this tonight. My echo is in the kitchen and I asked it to play music (Radio 2) in the dining room on my Play:1 - it worked well. Until I asked the kitchen echo to play Radio 2 too and it was out of sync
The Play:1 can be bought for £180, so it appears to make (a little) financial sense for those already with an echo/dot to buy the Play:1 and not the 'Sonos One'. Although, I can't see how they can continue to sell the Play:1 and the 'One' (they don't make it easy do they?!) so similarly priced.
For casual, background listening though, I think I'd rather buy two new Echos than one Sonos Play:1/One.