So not enabling this by default it's a good choice, Spectre is very difficult to exploit: so if you do critical things you enable the fix, if you use the computer for gaming, video rendering, and things where you don't care too much about security but you care about performance, you don't enable it.
The CPU can not be allowed to disable the fix, because then that could be done by an attacker. Therefore the only more secure way is to move in the secure direction, from insecure to more secure.
If for some reason you even want to forbid even privileged code from modifying the config then add an other "lock" bit that forbids subsequent reconfiguration till the next reboot.
And if an attacker can run code at the kernel level it's a non issue, as they're already on the other side of the airtight hatchway anyway [1]: they're in control of the computer and the memory.
[1]: https://blogs.msdn.microsoft.com/oldnewthing/20060508-22/?p=...
From what I've seen. There's been demonstrated attacks using Javascript in Chrome to dump the saved passwords from the browser using these bugs
If an attack is that easy to pull off, I don't think it's reasonable to make it an "opt in"
Safari/webkit have since rolled out mitigations to prevent the attacks that they figured out but it puts the lie to the idea that Spectre is only a theoretical attack that we've yet to see an exploit for.