Among average to poor hitters, the breakeven point is that much lower. Whereas the breakeven point for a great hitter is 45% to 50% success rate on bunts, for an average hitter, it’s all the way down to close to 40%, and for a bad hitter, it’s around 35%. And, we’d expect average hitters to be able to bunt better than great hitters (because of experience), and similarly, the bad hitters may be the best bunters (because they need to learn whatever to survive as hitters).
So, to shift against an average or worse hitter is about the worst defensive alignment you can imagine, and the average or worse batter needs to bunt any chance he gets, when the bases are empty.
Tango makes the case that the smartest counter to aggressive infield shifting is to bunt against it every time. It’s an interesting read, though I wonder if the expected success rate for bunting for base hits, even against shifts, is as high as he suggests. At the very least, more frequent bunting against shifts would likely make defenses more hesitant to use them, allowing the hitters in question to go back to swinging away.