"It's pretty clear that the disciples thought that Jesus would return during their lifetimes, and this looks like a promise to that effect."
Ye translates the Greek word umeis [transliterated without accent or breathing marks]. It is plural. Jesus is talking to Jewish people in the context of their culture and ethnology. There will always be Jewish people, including the time He returns.
That's got to be some kind of a record: two years between Q and A.
Your answer makes a lot of sense and would explain the apparent contradiction. Has it been an accepted exegisis down the centuries?