And all these points that you are making are very similar to the ones that I have had to make to my non-patriots friends about this deal. They feel, and even one of my fellow 49ers fans feel that bill pulled one over on us, and they bring up Cassel and Mallett.
To which I tell them: Cassel was sent to Kansas City. Mallett was sent to Houston. These were direct rivals of the Patriots. If they really felt that either one of these two would be a threat to them, would they have sent them there? Of course they wouldn't have. It is being said on talk radio that one of the other reasons they moved him to the NFC, and to the NFC west, was they felt that jimmy could be a really good QB, and they wouldn't have to face him but once every 4 years or the Super Bowl.
In general, Pats would have always rather traded Jimmy to the NFC. This being said, there was also the element of 'value' they may or may not have been offered by any AFC teams. For example, had the Browns offered multiple 1st round picks, I doubt he will be with the 9ers now. Chance are, if they had competing offers, none was egregiously better than that of the 9ers. So, when the offers are close enough, of course the Pats would rather see him off to the NFC than the AFC.
There are. of course, AFC teams with which I doubt the Pats would even consider a trade like this, at any price - e.g. Denver, Pittsburgh (with Big Ben semi-retired), Houston (before they got Watson). These are teams that with a potential franchise QB like Jimmy, could well become SB contenders for years to come.
When it comes to the history of Patriots back-up QBs failing elsewhere, people often point to the Patriots system and other such BS. In general, teams that need a QB suck, have under-manned rosters, and there will always be a big step down in coaching when leaving New England for just about any other team in the NFL.
So, of course if you took a decent but not great QB (eg Cassell) away from a top team like the Pats and put him on one of those likely inferior teams, he isn't gonna light the world on fire.
This being said, neither Hoyer, nor Mallett nor even Cassell (made the pro-bowl in his first year at KC) are in Jimmy's range, talent-wise. Aside from mallett, who I always thought was a huge miscalculation by the Pats as he didn't fit their offense, all other Brady back-ups were late-round picks or UDFAs.
Basically the idea was to pick up an average Joe, coach him up to be just decent to back up a HOF QB still in his late 20s or early 30s and that was it. They always knew with those guys that they were good enough to replace Brady for a game or two, if needed, but not much else beyond that.
The Garoppolo pick was different - they spent a 2nd round pick on him, in a very talented draft class, at a time when Brady was soon to become 37 yo. When they were making that pick, they did it with the idea that Jimmy was gonna be groomed to be Brady's eventual replacement and face of the franchise. No one could imagine, let alone realistically project back in 2014 that 3-4 years later, Brady would still be playing at this level at 40 yo.
In short, Garoppolo is on a completely different talent level compared to any previous Brady back-up and most importantly, the Pats had a very different plan for him when they drafted him in 2014 and it wasn't for him to be just "decent enough" for a game or two.
With Shanahan he will be in a system that fits him perfectly. As long as they build-up the talent around Jimmy to a respectable level, I would be shocked if he isn't widely regarded as at least a top-10 QB in 2-3 years from now.