More batteries, more likely that you'll have even just one of them fail. Since even one of them (to your point) failing is enough of a reason to divert the flight, better to start by reducing the probability of that happening in ways people can swallow.

So having 500 batteries on board is okay.. but 750 is too risky? I just have a hard time believing that the math is actually mathing in this case. Maybe you're right, and this is just a first step to get people to gradually accept more restrictions.