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.

Public companies obviously intentionally want to make everything as shitty as possible, just to extract money, but lets accept the hypothetical that subscriptions will actually be banned. Wouldn't that be great?

You would basically be treated the same as Tier 1-3 ISP's, pay for the cost of the routing to the company. That phone plan that costs ?? €/$ a month becomes "Pay as you use it". Flat fee per gigabyte / message etc. These plans were at least here in Finland, and I think my phone bills were around 4-5 EUR a month and a cap that you cannot exceed that month, though smartphones and data plans weren't a thing. Now everything is a subscription.

Now back to hardware vs software. You obviously pay for the software also when buying the hardware, but for whatever reason the user doesn't own any kind of rights around it. This has obviously become much worse the past few years (TV's have ads etc). I really don't think that the issue is anything you listed, the issue is that greedy companies want to use the subscription model rather than play fair. Phones and modems are EOL at best in a year. I have a PFSense router that cost me less than a router from my ISP used and it's EOL and security is something I don't have to worry about.

Modems and routers have most of their features dedicated to home networking and are not usually made by the ISP. Them connecting to the internet is one of the smallest features they have. The other features are related to offline networking and tight security, you can actually just plug an ethernet cable to the wall and get connection from your ISP. Same as using a modem and putting it in "bridge mode", which will completely bypass the features of the modem/router.

The issue here is that the companies don't want to provide value, they just want to extract as much money as possible, which is wrong. Laws and regulations are desperately needed and even something as radical as banning subscription services for user devices would be a net positive. Renting Tier 1-3 operator infrastructure for your router/modem to work is completely different than "You have the device and the software, but we block you from using it, since you don't pay", which in my opinion is ransomware, not subscriptions.

For right to repair and owning these devices, I completely agree with you.