He has been rebuilding. It takes time and discipline. Every other team, with far lower highs, has bigger dips than the Patriots have had so far on this downswing. The league is built to actively punish success and the team had been borrowing from the future (at a sustainable, not reckless pace) for two decades. You don't rebuild by spending through the roof. You rebuild by cleaning your books, establishing a base (which he has, by focusing on a young, deep defense that stayed very good despite their two best players hitting IR early in the season), and don't start borrowing again until it puts you over the hump.
You can't sustain borrowing. Bill did it longer than anyone else by a large margin, by keeping the numbers manageable, but it's why Brady ended to begin with. Two decades of your players costing more, your draft picks being lower, and continued cost of paying back borrowing added up until building a team could no longer be done. Nobody rebuilds overnight. There are teams that are perceived to do so, but those teams that cleaned house and became "instant contenders" have a several years of top 10 picks for the new guy to build around.
As for Mac, it's entirely possible he's not the guy. But he didn't start to break down until repeatedly getting shredded on three step drops. The first several games of the season, he was playing great football, consistently making the right reads and putting the ball in the right place at the right time on any play he had blocking long enough to complete his drop. Game ending drives weren't ended by Mac making bad decisions or throws. They were ended by a churning, heavily injured receiving room not bringing down his throws. Eventually, after half a season of that and getting killed on short drops before guys could win short routes, he started to press and make more desperate, dangerous plays. It's possible we broke him. But unless you absolutely love the guy at 3, quarterback isn't the priority. You can't develop a QB behind that line. You invest premium assets at line so you can protect someone. Building a team isn't overnight. If you draft a QB without protecting him, you're just going to break that guy too.