The length of time it takes to rebuild in the NFL is regularly overstated. Some teams get stuck with salary cap problems that take years to dig out of, others set themselves back by trading off too much draft capital, but typically NFL teams can turn around their fortunes fast.
The demise of the New England Patriots was real. Tom Brady left. The roster had as little talent as any in the NFL by the time Bill Belichick's unprecedented run with the Patriots was done. Last season, the Patriots suffered through a 4-13 season with a rookie head coach they obviously regretted hiring.