So what use is a consumer modem without an internet service?
You can still use it to network with other computers over the telephone network. Heck you don’t even need to do that via the actual telephone service you can just run some wire.
But I think what was actually meant by OP is “tied to a specific subscription service” as well as “disables features that don’t need a subscription service when you aren’t subscribed”.
Phones, arguably, don’t perform their primary function without cell services.
You can use them as e.g. smart home remote. The cellular modem is going to go unused (at least apart from emergency services) but that’s only a small portion of the hardware, and modems were only ever locked to subscriptions (at least over here) if the phone is subsidised by that subscription. I don’t think they even do that any more, they replaced it with minimum contract durations. In any case even back in the days you could unlock it after some time or coughing up some money.
that’s why I think it should be root-able, serviceable, and speak in standard open protocols
Yep I wish rootability was included in the new EU regulations, it would solve so many issues at once. OTOH: It would solve the issue for people who are tech savvy enough to do such things, gotta be careful with our own elitism there. Enjoying consumer rights should not hinge on being a grease monkey.