I repeat, features of a product cannot go away.
They can, and they do, and not just on Apple products. But fortunately, this isn't an issue of a feature going away for good. You can update your software and regain the feature.
Again, a product I bougjt with X features must maintain such features all it"s life until it dies.
I thought that this was already known.
It's not known, because it's not true, and absolutely
never guaranteed when a service requires infrastructure outside of your local hardware to function. FaceTime and anything iCloud happens to be such a case: if Apple for any reason decides to end the service, or even if it deliberately chooses to discontinue supporting older hardware, or if they happen to go out of business tomorrow, then that's that. A lot of iOS features will stop working, and the functionality of your device will be greatly reduced.
And again, Apple is not the only one. Try getting an old 1980s cellular phone to work on today's cell networks. Spoiler: it won't. There were lots of videophiles who bought into the notion of
DIVX as a home video rental format. Those discs are now useless.
There are lots of examples where produts that rely on services to function have stopped working even though the hardware is perfectly fine.
T-Mobile's sidekick, the
Helio Ocean, and
Microsoft Kin are all products that don't fit your ideal of the features lasting forever. there are many more out there, and the list will continue to grow.
Even on a more basic level, this is a problem. Your computer and iDevices all rely on your ISP and cell carriers agreeing to support your device with specific standards. If they choose to go with different technology (and they will, eventually) those devices WILL lose access to the internet. Film cameras may still exist 30 years from now, but there's no guarantee that there will still be film to take pictures with. Someday, maybe decades from now, the car you drive may no longer work even if you manage to maintain it in pristine condition, all because we've run out of fossil fuel.
Fortunately though, this isn't one of those cases. Update your OS, and FaceTime comes back.