Did people by these connected devices and really expect them to last forever?
No, I never expected them to last forever.
But I did expect them to remain connected with each other until they failed. That, after all, was the whole point of difference with Sonos.
I thought they did keep working, but with no upgrades and may not be compatible with newer models. If that's whats happening then what you've got will keep working or have I misunderstood something?
They will keep working, but will need to be on a separate network to the other Sonos speakers. Given that Sonos has marketed on the key premise of a single connected network, it's kind of a big thing.
The other option Sonos has offered is to keep a single network, but hobble all speakers from any future updates (which is probably what I will do). How this will work in practice will be interesting as, to date, Sonos bombard you incessantly if speakers or apps are not running latest upgrade.
Sonos could have simply introduced a new range of speakers that were not compatible with older speakers - this would have given consumers choice. And they could have kept selling the older line as well for existing customers - a bit like what Apple do with the iPhone. However, what they chose to do was to unilaterally remove the most important function from existing products.