They run a routing/cross-auth system so you can stream from other IP-based audio services directly to your speakers. They aren’t entirely a hardware company and those integrations are a value-add for a lot of consumers. I think you should be able to run them in some kind of offline mode, though.
Logitech Squeeze players also had Squeeze network, which cost nothing, and is still in service today.
You can put a DAC on a RPi, install squeezeplayer, and attach to the squeeze network for free today. I have done that exact thing in the last six months.
They at least need an update server so they can receive security patches. Once you need that, it's a slippery slope to depending on lots of things in the cloud.
Open source server which runs locally. The hardware is long since discontinued (but plentiful and easily available on craigslist etc) and it can never be obsoleted as everything runs locally.
Plus Squeeze Network (still free, still working) for Pandora, Spotify, or other network services.
Have you tried squeezeplayer on a RPi with a DAC? It works great. I have one alongside my original Squuezebox. They sync perfectly for multiroom audio.