I would assume the theory gained fuel after Omniverse introduced Ben and Kai as end game. Even still, it still does not make very much sense.

For one, Paradox does not seem like the type to let Ben meet his future self often if that was the case. It would create too much risk of a horrible future, in my opinion.

More importantly, it is never really stated. I watched the Ben 10k returns episode and Paradox explicitly states that the overworking Ben 10k we first see is was an alternative timeline from the beginning.

Even with ultimate Ben 10k and OV Ben 10k. Ultimate Ben 10K is still Ben's future, I don't think that changed. I believe OV 10k and UA 10k are the same. Rather OV 10k is the later version of UA 10k as both have memories of the current Ben we know.