The Bills will always be competitive with Allen, McDermott and Beane, BUT that doesn’t come without its limitations. There is a window of oppprtunity and the Bills are in it for this current squad.
There will be staff and roster turnover to deal with in the coming years that will present its own set of challenges.
Both the Bills and Chiefs seem to be in excellent positions with the QB/Coach/GM trio and while I don't believe that Patriots-like longevity is realistic, we're talking about 2 of the very best talents at QB that this league has ever seen, so I guess we'll see.