Obviously the quality is already very high, but still quite a ways to go for each batch consist of "50 AirBnBs", which to me is a far more impressive goal than having 1000s of lower quality startups. It's also better that YC is a signal of success than "not-YC" being a signal of failure. Franchising would dilute the brand and more importantly be a distraction from (as YC would always say) making and improving something people want.
YC is relatively "open-sourced" and there's nothing to stop others from creating accelerators with similar models around the world (and in the valley). There are already hundreds of imitators and these catch many of those that YC misses.