Your math works out, I think, assuming of course the estimates about debit activity and whatnot are in the ballpark. To put it another way, if they lost 24% of their debit activity they'd make the same as they did without the cap and the $5 fee. This formula doesn't account for revenue lost by customers closing their accounts altogether instead of just reducing/eliminating debit card use, or the effect of bad press on their stock prices, but without those factored in they stand to increase their rate of profit quite a bit, so that probably evens out.

So what basic principle does this boil down to, though? To keep the bank from suffering and horrible things happening, they need to not merely profit but make a profit equal or greater to last year's, every year?