7.1 is the sharpness sweet spot of most APS-C kit lenses
That really depends on the design of the lens. There is no rule that 7.1 should be the sharpest. But even if is, the increased noise would compensate. My point is that if Fuji can make a 18-55 2.8-4 that is tiny and cheap, Canon should be able to make a small lens with 5.6 aperture.