Yes this did cross all of our minds. I was specifically assigned to fly with a new FO back at the regionals because management wanted a second opinion, post IOE, on the guy. He had been a 20-year ASEL CFI and after a divorce decided to run away and join the airlines. He struggled significantly but had a good attitude so I (barely) recommended they retain him. No idea what happened in the long run. Career stagnation or complacency can indicate potential difficulty succeeding with the next step. Either because the stagnation was caused by training struggles up to that point and fear of greater challenges, or complacency which could mean difficulty shifting gears back to student after so long.

I'm less concerned about the OP, given his penchant for learning new things and the martial arts background.