How are you comparing what Brissett is or isn't, and what Garropolo is or isn't?
Your insurance and longevity argument assumes a great deal, as Jimmy was knocked out of play in less than 2 games after working as a backup for how many years? Brissett, as a boot rookie, played the same number of games and stayed on the field with an injury when the Pats needed him that placed him on IR after Brady's return. If Jimmy is good for 1.5 of every 4 games, that is little insurance of anything. We have no real idea of his durability, so he is a huge question mark.
As for Brissett, some major personalities, like Parcells, think the world of his play, and Parcells for all his faults is not awful in judging talent. With an actual year with the playbook and a second preseason, are you betting against him? How long did Brady have before he stepped in as starter?
Jimmy is either fool's gold, in which case trading him is a plus for the Pats, or he is the second coming of Tom Brady, which will be realized potentially years down the road and/or by forcing Brady out in deference to this career backup. If Jimmy is as good as you believe him to be, then the Pats will have to pay him handsomely to keep him around until Brady's career expires. Absent Brady's arms and legs falling off next year, I am not sure this Armageddon scenario has legs.
The Pats have been and always will be next man up. Belichick follows a cost-benefit assessment for every paid player on the roster. If he can get a crop of future starters for a player likely walking one year later, then I see that as smart business and adios Jimmy. If there is no such opportunity, then Jimmy stays and earns a comp pick, provided the Pats don't view him as the cost-effective option before free agency.
This notion the Pats cannot live without some Jimmy seems to overlook the reality that for all but 1.5 games, they have done just that. Many of the so called heir apparents have passed into the light over the years (Cassel, Hoyer, Mallett, etc.), and the Patriots war machine has somehow thundered on in their absence. Trying to exalt JAG's departure to a calamity assumes far more than the evidence supports.