You just have to look at the phone community (XDA Developers, etc) to see how this will (eventually) go.
A good example were the old Asus Transformer tablets. They were a super niche device, but it still lended itself to Linux and so a small team of people managed to load Ubuntu on it.
Another are Samsung phones. They try to lock people out, but they have popular enough devices that people find a way to put LineageOS on them.
Finally, even iPhones aren't immune. Small teams of people have managed to load Android on them and get it (partially) working. More people would give them even greater functionality.
If laptop manufacturers lock things down with ARM, there will be people who work around those mitigations and install their own OS on that hardware. Tooling will be developed to make that process easier and easier for the next round of people with that device (or future devices). It'll suck up front until the community grows large enough to work around issues faster and faster.
And that's even supposing worst case scenario. I'm not fully buying the idea that you _won't_ be able to change the OS on these laptops. Microsoft has tried (and failed) to lock other OS's out of their laptops. Chromebooks are (currently) the largest market of ARM laptops and you're able to change the OS on them. Apple might be the only company even remotely able to hinder freedom on their devices.
Either way. In the war on general computing, I'm generally optimistic for the users.