It seems to me that Belichick has never failed in rebuilding a certain squad on the team, when it is his entire focus of attention.
The secondary needs 10-12 ball players to rebuild, and that has to be a multi-year undertaking, and it has been. But he rebuilt the TEs in a single year as well as the RBs, and the O-line took a couple of drafts to find the bookend tackles, but everything else fell into place after that, in a single year.
This year he aimed at the WRs, because he knew the veteran patches and Shadow Roster pieces were worn out.
He is succeeding spectacularly. Why is everyone surprised??
It depends on how you define failure, as well as 'entire focus of attention'. He overdrafted and/or outright missed on tight ends for nearly a decade before he nailed it with Gronk and Hernandez. From 2007 (when Seymour left) to the present, he's been extremely hit or miss on drafting defensive linemen to replace the talent of the guys who are gone, and to date hasn't succeeded. His track record drafting safeties post-Wilson has been pretty bad. Whiffed on Chung, Meriweather significantly underperformed his draft position, Wilson has been bad. Hopefully Harmon will reverse the trend, because to date the only pick who's really worked out was McCourty, and he was drafted to play corner.
Speaking of corner, that's been a mixed bag as well. Dennard looks like a great pick, and McCourty's definitely a hit in my book, but prior to that there were a whole lot of misses.
That's not to undersell how great the Pats have been at re-stocking the depth chart at OT, RB, TE, and hopefully linebacker. But to say that they're batting 1.000 in these situations is simply wrong. This is classic confirmation bias at work; you're remembering the instances that validate what you're saying, but you're forgetting how many years of misses went before those hits, as well as the instances where the hits still have yet to happen.
Even where they do hit, sometimes it takes a long time and a lot of misses, If the WR retooling project plays out at all like it did at safety, TE, cornerback, or DT, then we're in for a few more years of misses before eventually landing on the right guys. Luckily for the fans, all indications so far suggest that this rebuilding project looks more like the OT or RB projects, where the players drafted to fill the void pretty much succeeded in bulk on the first attempt.