Not much point in waiting for OEMs to update to 4.2.2 before launching 5.x since the whole process will repeat with 5.x and subsequent releases... unless Google wants to make a statement that it wants to force OEMs to update most of their devices to the highest eligible version available before giving them access to new versions for future devices, which makes sense since it would reduce the number of different base build versions in the wild Google and app writers need to deal with.