One thing to note- the 49ers back in the 90's with the West Coast offense and the Patriots today remind me of each other with regards to QB. The 49ers had what Walsh called a "complete system," and as long as QBs were sufficiently trained with a year or two of learning, they seemed to be able to turn modest QBs into good QBs, like Elvis Grbac, Bono, making Jeff Garcia a Pro Bowler.
Similarly, the culture, teaching, coaching, and "read the defense" passing system of the Pats may be similar. There might be an easier transition window for the Pats to develop a college QB into someone who can take them far, like Jeff Garcia, although not be at Brady/Montana/Young levels.
That's what the Pats system and current winning culture and coaching strikes me as, although I guess my only solid evidence is Matt Cassel looking good here and pedestrian elsewhere, except he had a prime Moss and Welker and a great O line. Ryan Mallet didn't look much better here than on the Texans I guess.
That said, if Garoppolo is special in the same way that Brady is special, there is going ot be an interesting dilemma in a year or two. I don't know how you release Brady (the ultimate "Patriots philosophy" decision of a year too early, etc") without being sure Garoppolo is the One, and the only way to do that is to see him in action. Which won't happen with Brady at such a high level.
My opinion is Brady has a great chance of playing to 45. I believe in the effects of modern nutrition and a vegan lifestyle with low impact resistance band training. If I were the Pats, though, if they're going this route I would invest in a couple high-end offensive linemen. Having a healthy Brady (obviously as you're all aware) can paper over not having the highest priced WRs or even defensive core and keep you with a good shot at making the AFCC/Super Bowl every year, but not vice versa. Meaning I think the O-line is good, but we need to be closer to great O-line even if it means sacrificing some talent elsewhere IMO to keep Brady from getting all these hits as he gets older.