I've made the same point earlier in this 50+ page thread: it is the GM's job to evaluate everyone on the team, and one of the few reasons it'd make sense for BB to "shop" Mac was if he decided he had hit his peak and there was no more upside available. But it is also part of the GM's job to "do what's best for the team", something BB says countless times. So, factoring in cost is not just a fan's point of view, it's also the GM's point of view. It's not either/or, IMO.
Evaluating production and cost at the same time is the reality of the job. If he moves on from Mac, he has to be convinced he has a viable Plan B where he can put someone who is an upgrade on Mac on the basis of production and cost. Given the fact most of us think Mac is a starting NFL caliber talent who is on a first round rookie contract, it's hard to do that. I don't buy the idea that Zappe is an upgrade on production, and I don't think the staff does either.
At the bare minimum, I think the "shopping" was never going anywhere because Mac has so much value. Even if he ends up being a backup instead of a starter level talent, it makes sense to keep him as a backup for at least the next two seasons given what kind of salaries backups get these days. And I feel the coaching staff realizes they haven't been able to put him in a position to perform at his peak over the last two seasons so they don't really know if he's peaked out or not.