This is not a good analysis. Anectodal reasoning is so tempting but it's just not very persuasive in baseball - there are hundreds of active pitchers and thousands of historic pitchers, you can find examples for every mold. I don't care that a couple of SPs succeed with s***** fastballs, or just one dominant offspeed pitch. Kuhl's odds of pulling it off are extremely low, just like every other pitcher in history with a mediocre fastball and a plus breaking ball - the vast majority of those guys are relievers. I mean you could argue that Kuhl is already performing beyond expectations by being an above replacement level starter.
Look, in 2020 he tried it. FB% plummeted to just 44%; he was throwing offspeed stuff over half the time. The result was the worst walk rate of his career, worst HR rate, and therefore his worst FIP.