…is it, though?

It’s highly likely that you had one or more bad-but-not-dead cables (like a weak termination) that was limiting your speed. By swapping everything out you fixed the problem. Cat 5e to 8 definitely shouldn’t have caused that much if a jump (if any).

Agreed on both counts. This happened because Microsoft made adoption easy. And this will be fixed within a day. None of the fundamentals have shifted. Even though it’s stupid, this isn’t going to fundamentally shake anything up.