Solution: You can now have Airplay 2 with any “stupid” speaker!
On a same day when Bose closed my previous thread New Airplay 2 multiroom support (coincidence?) Apple released a firmware update to their discontinued Airport Express router introducing AirPlay 2. With AirPlay 2 update, 2012 Airport Express models offer support for multiroom-audio. AirPort Express features a built-in AUX port that lets you wireless stream music to a connected speaker over line-out. Airport Express routers are discontinued and Apple does not sell them anymore, but you can get one from popular online auction sites for around 40 bucks.