Other incubators/accelerators have focused on companies at different stages, but none that I know of are expanding, as YC is doing, from beginning to middle (to end?). This makes sense from YCs perspective because anything with network effects increases with more people/cos. Additionally to borrow a phrase from a16z, YC is becoming a full-stack or bundled company: Go to YC to get your start, your growth, and perhaps in the future, your IPO, who knows.