...it's one where buying a new toaster means replacing all your lightbulbs...

I think in such a scenario it's far more likely that the bulk of people are going to say "screw it, I'm going to toast my bread the old-fashioned way, get back to me when this rolling fuckup is over".

If the companies involved want to build a bigger home automation market, they're going to have to wake up to the fact that they need a great interoperability story.