Yeah, but people don't really want to use the BSD userspace. A lot of the Linux stuff people want to build on assumes a GNU userland and it's not trivial to build a BSD/Linux that actually does relevant computer stuff.
But in places where that stuff isn't relevant, we already see a lot of locked-down devices like the Nintendo Switch and PlayStation based on BSD precisely because they can leverage free software but still lock it down. macOS with its BSD userland is also kind of like this -- the OS is getting gradually more locked down over time, but the frog boils slowly.
If you tighten the screws too hard and fast then people will scream and yell and maybe leave your business for a competitor -- even though it's technically feasible, that means you can't disallow access to banking websites for generic-browser-on-generic-OS now. But we are, brick by brick, building a foundation where that will seem inevitable.
The argument is basically that making it easier to lock down general purpose computing devices like desktop computers (by, for example, making a non-GPL drop-in replacement for GNU *utils) will eventually aid in making it happen. The powers that be will use tried-and-true arguments about security and think-of-the-kids etc to make it seem like running a mutable, untrusted OS is an unacceptable risk.