I would personally prefer one step further and force to remove all locks if the customer asks it, it's their device, they are free to do whatever they want with it, it's both the right approach morally but also for environmental issues.

They could but it would cost more than most users bargained for. Most likely a manufacturer would offer you the open device for the price of the device plus the expected loss of value from connected services (maybe something on top just for good measure), or a locked "subsidized" version that can or has to run only as we see today, and that will cost as much as one does now. And they could easily make a case for this even in court.

But once the support ends it's more or less implicit that the manufacturer abandoned the device and no longer expects profit (or expenses) related to it and keeping it locked is a harder case to make.

And I get a manufacturer's desire to profit from their own devices. But once they abandon them I see no other reasonable legal or moral claim a manufacturer can make. They should issue a final update that removes any protections, and make all relevant drivers and documentation freely available (or even for a modest one time fee) even if this means some binary blobs and a bunch of docs for how to interface with them.