Disrupting the Clovis paradigm is definitely upending in a field where new ideas take root by retiring the past generation of professors.
If you postulate pre-Clovis settlements, now you need to explain why they weren't able to quickly expand to fill what was very fertile land for hunting, the way the Clovis people did. And how they managed to get so far south without having populations large enough to leave any unambiguous artifacts.
From the outside, I like Jared Diamond's article about it: https://www.edge.org/response-detail/27111
These "upending" discoveries of pre-Clovis sites remind me of the frequent news articles that were being written about the EM Drive which purported to revolutionize space travel by violating the fundamental law of conservation of momentum.
I'd imagine there's plenty of possible reasons pre-Clovis Americans wouldn't have been as numerous or left as much evidence as Clovis-people did; most obviously, that the continent was much colder, and therefore supportive of much smaller populations, than it was in the Clovis culture era- particularly in the northern regions, where it's often objected that they "should" have left remains. Obviously, this isn't an argument that there were pre-Clovis settlers, but an example of why the idea can't be objected out-of-hand through "common sense."
Okay, but we're assuming a population capable of migrating as far as San Diego, and capable of hunting mammoths. You'd need enough population to sustain the many hundreds of years of generations it would take to get that far, and just exactly the sort of population balance where they wouldn't leave any obvious traces? And given they made it halfway down the continent, and through the most inhospitable places, why didn't they go further south where they could have immediately flourished in lush lands full of megafauna who hadn't evolved to fear us? Humanity's record seems full enough of such population explosion incidents after all, including the Clovis-people themselves.