If Jimmy has to leave NE, we need to find ways to balance the risk of never getting a QB as good as Garoppolo again (Jacoby fails to develop / we can't draft the next Gropp), by building perennial, dominant defense + OL. So the compensation any buying team would have to surrender should result in 3 additional franchise players let alone the players to be drafted with our own picks : high 1st rounder (tradable in multiple early round picks) + player, or high 1st + 2nd in 2018 or 2019, or multiple early round picks in 2017/2018 or 2019.
If no one want to pay that price, then we have to convince Garoppolo that he is Tom Brady's Steve Young and give him enough guaranteed money in a long term/backloaded deal with big playing time incentive, just in case he has to take over earlier than expected (easier said than done).
By the way, once our own guys are extended or franchised, we should have enough cap space left to sign at least one top Free agent (if the money is right).