I don't think Josh, or the Bills in general, are unstoppable on short yardage plays... and it's even harder that close to the endzone as it's more difficult to find space. If the Bills struggle to convert 3rd/4th & short when there's a far more space behind them, how will it become easier when the entire defense is packed into one crammed area?
Either way, league average is 49%, so even if the Bills hit that average, they'd break even had they just gone for the PAT. It's just not a meaningful gamble unless you somehow develop a strong power running game to at least threaten the opposing defenses with. At this point, every team knows Josh is a good runner, so they prepare specifically to address that.
That's why on those short yardage situations, Josh rarely has an opening, as every team is waiting for exactly that (and why you get weird sequences like against the Rams where he tries to scramble, & defenders are immediately in his face, resulting in him trying to chuck up a pass that just got batted down).