I wouldn't say the D let them down in the 4Q of the Eagles game. They very consciously played prevent, forcing plays to remain in-bounds and short-yardage, keeping the clock running. They were playing to the score, and it worked out (famously) well. If anything, they gave up a TD
by design, after a 13-play drive. If they absolutely needed a stop you'd have seen very different defensive calls.
Likewise, any critique of the defense for the Rams or Scottish games that focuses on the 4th quarter to the exclusion of the awesome job they did in Qs 1-3 is kinda cherry-picked.
Not that your point is undermined in the slightest, of course: the D did indeed rise to the occasion in epic fashion vs the Seahawks and Falcons. I'd argue their 30+ minutes of shutout from early 3Q through OT is one of the most impressive defensive efforts in NFL history, right up there with
other epic struggles BB had a hand in. But in so doing, let's not shortchange the incredible defenses from the earlier generations of the Belichick Patriots.