I can understand 15 year old equipment not having the processing capability to deal with their latest software, but, I agree, that a 5 year old Sonos unit being obsolete is rather odd.
What it most likely means is the Sonos was using the same platform for a decade with no upgrades to the processor what so ever.
While doing that is the best thing for the company because it allows them to maximize profit, it's pretty shabby for the users.