Edge is just being honest as always. He and the rest of the group are always second guessing their abilities to see and maintain the ultra level of "customer satisfaction" they target.
Bono has said you'll never get, " ...2 crap albums..." from the band , and that is of course based on their personal veiw of the work not the success dollar wise.
It is this integral fortitude combined with their talent and love of music that will keep them going until they decide its time to move on. While that would be a tremendous loss, it is their perogative.