Actually the idea is quite interesting
U2 have enought material hidden in their vaults for making a 3 hours long film if they wanted it.
I'm not talking about doing interviews for an upcoming DVD release (like Bob Dylan No Direction Home for example), they don't even need to do that. It's quite simpler!. Since the early years they have been doing interviews and documentaries for TV (remember 1981 Boy Group, 1982 Riverboat President, 1982 European Festivals, MTV Fast Foward 1983 documentary with the band, 1984 Slane Castle, 1985 Wide Awake in Dublin, 1987 Outside Is America, 1989 Sydney Special, 1987-1988 outtakes from Rattle & Hum, Wild Irish Rose 1990, Zoo TV Outside Broadcast 1992, A Year In Pop 1997, Sarajevo 1997, MTV icon, etc, etc ) so the director would have to get the interviews at that time, add U2's music, live footage, promo video clips and more recent interviews, and that's all !
Actually it's a more interesting thing to watch a documentary filmed on those days than a documentary like Beatles Anthology, you know, the present band talking about the past with footage from the years
. I can't think about any documentary without present band or artist doing interviews to remember the past, that's something predictable and cliche
It could be far more artistic and interesting to make a new kind of documentary films just with footage and interview from those years instead