The thing you are missing is that good teams choose a backup and a third stringer in the off/preseason and by now they have a good idea what their backup can and can't do within their offense. I remember distinctly when Mahomes got hurt in the playoffs against the Jaguars and Chad Henne came in and let a perfectly competent TD drive, as a recent example. If Gabbert or K Allen have to start three-four games for KC / Buf they're not going to do what Mahomes / J Allen can do but the coaches aren't going to ask them to.
These teams have no idea if Jones can grasp what they want to do or if he'd really be any good at it -- or better than the guy who has been practicing with them all year. And they'd need at least a couple weeks of practice to figure it out. They're going to stick with what they have and the guy who's been practicing with the team in almost all cases other than complete desperation.
Your last sentence is my point. Desperation happens a lot in the NFL. Last season, the Rams started 4 different QB's. Now they were never really in contention, so picking up Mayfield off waivers and immediately throwing him into the game wasn't a big deal. But if they are still in contention and Stafford goes down, and they quickly realize that Brett Rypien or Stetson Bennett are not the answers (or if one of them gets hurt), are there a ton of better options out there?
Whereas when Justin Fields goes down again, like he did on Sunday, Chicago is going to be perfectly fine playing Tyson Bagent rather than give up any assets.
Like I've said a few times, I'm just pointing out the only situation where I can see Mac having any more trade value than like a 5th or 6th rounder. It's hypothetical, I'm not saying it will happen. Let's say Mahomes does get hurt, and Blaine Gabbert comes in and then he goes down or he completely sucks, which he's basically done his entire career, do the Chiefs just resign themselves to sucking, or do they make a move? Who knows, but it's the only way I see Mac generating trade value this season.
I mean, if it were true that all teams planned for their back up and 3rd stringer to be able to step in, what's that say about BB and the Pats QB options right now?