One day I wondered what happens if it can see two paired devices - does it forget one? Is there some unusably awful UX? Does it brick them altogether?
So I paired the Linux laptop. The dongle (which says "Connected" when I turn it on normally near the phone) now says "Second. Device. Connected" when it can also see the laptop. It appeared as one of the output options in the laptop's audio settings. If you play music on the phone, or receive a call, the music on the laptop stops, and you can take the call (or listen to music) instead.
Essentially, with the cheapest available product it appears to be as you say "completely painless". What exactly are Airpods / Apple doing here that makes it worth the extra? Maybe the voice telling you what's going on is nicer?
They could if Apple would license out the W1 chip, or even sell dongles with W1 functionality that convert the 3.5mm jacks in headphones into bluetooth (like one of these: https://www.amazon.com/Bluetooth-Receiver-TaoTronics-Hands-F...).
Of course they never will, but a man can dream.
now i know