The Patriots also benefited from YEARS of cupcake schedules. The rest of the AFC East was so bad for so long (and the Jets and Dolphins largely STILL are) that the Patriots practically began most seasons at 6-0 by default.
Belichick is one of the all-time great HCs, but I still wouldn't put him above Lombardi or Walsh.
The Steel Curtain defense basically got old all at once. As a result, the Steelers started going downhill a good 2+ years before Bradshaw ever retired. It also didn't help that the draft magic was no longer there - check out some of the Steelers' drafts from the '80s and be prepared to cringe. Hell, Dan wound up firing his own brother it got so bad:
10 Critical Dan Rooney Decisions that Shaped the Pittsburgh Steelers (steelcurtainrising.com)The quality of the Steelers drafting took a nose dive in the latter half of the 1970’s and Pittsburgh’s drafting didn’t get any better as the Steelers drafting position dropped as trips to the playoffs became rare in the 80s. There are a lot of reasons for this, and it would be horrendously unfair to scapegoat Art Rooney Jr. for the decline.
But it is also clear that by the mid-1980’s Chuck Noll and Art Rooney Jr. could no longer effectively function as a team.