My thought exactly. If someone is already able to replace arbitrary DLLs on the system (a precondition for this "exploit" to work), all bets are off, and their ability to bypass driver signatures is probably the least of your concerns.
Downgrade protection is quite a hard problem to solve without at least breaking the automatic recovery tools. In theory Microsoft could register the versions of every system file somewhere, sign+hash that, and store it some place secure, but then they'd need to deal with reverts of system components after a failed update or restored system images becoming unbootable.
In practice, I don't see the use case for this attack, though. You can just put a legitimate, signed driver with a known exploit inside your executable and load that. Microsoft chooses to keep unpatched, vulnerable drivers rather than break hardware support for millions of users.