The Pats have a decent chance to win another Superbowl with Brady. They have little chance with Stidham. The QB position is different from any other position. You can put a lesser player in a position to succeed at a bargain price at any other position. Putting 13% of your cap to QB is not a deal-breaker, but it depends on the rest of the team. I'm going to look at some numbers here to test your premise, Tune. Let's assume the Pats sign brady for 2 years at $52M, cap numbers $25M in 2020 & $27M in 2021. That leaves them $22M (not much) to resign free agents, but there are other ways to save money.
HT has an $11.4M hit but can save $8.9M by cutting him. He's a great player, but the depth at LB is far better than it is at QB, and he's been lost for many games in the past and might be again. If you cut him, it's like signing TB to a $16M/year deal and needing to bolster the LB corps with mid-level players. That's something that this team has done before. I hate to see HT go, but not as much as I'd hate to see Brady go.
Sanu has a $6.5M cap hit which can all be saved if he is cut. Bye bye.
The Pats now have $37.4M against the cap and the best QB of all time. They'll need to hold onto $7.4-10 M of that, but we can try to get that back with some mid-level cuts or a restructure or two later on in the process. Let's see what their team looks like and what they need to go get, starting with their positions of need.
WR - Dorsett and Sanu are now gone. The team is not much worse for it, but they need to sign a free agent here and at TE, while bringing along the rookies. This will be a chunk of change, and we can come back to it later.
OL - Thuney and Karras are FAs. Resigning Thuney is going to cost at least $10M per year. Much of the hit could be pushed off until after Brady leaves, however. Let's say 4 years at $40M with $10M as bonus and $5M in initial salary. Karras should be modest money $2M per year. That's a $9.5M hit, leaving them with $28M. I'd draft an OL here, and assuming Andrews comes back, they have a serviceable and deeper OL than they had this year.
DL - Shelton, Calhoun, and Butler are all FA. Butler needs to be signed and Shelton needs to be signed or replaced. That's probably $4-5M between them. Let's call it $5M, so they can add a another fatty at a vet minimum. We're at $23M. They'll draft someone here.
LB - With me sending HT packing and Roberts, Collins and Van Noy as FA, this position is suddenly the weakest of all, and they are going to need three players to complement Bentley. (Bite me; this is hard.) Van Noy is in his final payday and will want >$6M per year for several years. I don't know if this is doable. I like his productivity and work ethic, but he's slow now and won't get faster on the other side of 30. Regardless, this position is going to need two (re-)signings and an early draft pick to shore-up. Let's assume that BB resigns Van Noy and Roberts at $8M combined for 2020 and drafts a speedy LB. Now, we're down to $15M and still don't have a shiny new WR or TE.
Secondary - Not a weakness, even with the loss of DMac, but maybe a place that money can be saved. Let's move on from the injured Chung and JMac to gain $6M, and give JoeJuan and Obi a chance to play SS. That's a bit thin, but now there is $21M to sign a couple of vet receivers and make the rest of the team work. (Or $15M to get them through the roster building process that runs through the first few weeks of the season with $6M left as play and patch money. A bit tight, but not out of the realm of the feasible.)
RBs are in good shape. ST could use a player or two with Slater and Ebner to sign, but that's all doable. My plan has holes, and the D won't be as good, but it's not as if the team can't compete like this. Expecting Stidham to be the answer is still wishful thinking at this point (at least from the information that we have outside the team).
Thoughts? There's obviously the option of extending HT, JMac, Chung, or Harmon to push costs to future years and keep them around. Harmon is a keeper, given his youth, and an extension might save a few bucks against the cap. I wouldn't be surprised if HT ended-up with an extension either. He could be* the best LB we've seen in this run, and his strength is his strength, so he could keep on for a few years if he can stay healthy. I think Chung could be ready to hang it up. He's the Gronk of the defense and has taken an unreasonable beating for years. That's a lot of talent to replace, but not unreasonable for the FA era. The only player that we can't replace is Brady, so I think he's here for another year or two.
