Buffalo had aging talent, lost to the Chiefs at home in the playoffs and lost both their #1 and #2 receivers. Meanwhile you have teams like Baltimore and Houston on the accent, and Rodgers returning. The Bills didn't even make the playoffs until Week 18.
Predicting a drop-off for the Bills was not unreasonable.
That being said, Buffalo bounced back much, much better then expected, and at this point the window hs been proven to be wide open.