Rivers and Palmer are old guys and, like the Patriots, the Chargers and Cardinals are likely looking at both today and tomorrow, with regards to the QB position. Trading for JAG (or any young QB) next year would put the Chargers/Cardinals in a position to use 2017 as their transition year, with JAG (or any young QB) installed as the full time starter by 2018. Tannehill's been a stiff, so swapping him for a decent QB out would be a net bonus for the Dolphins, even if that means Tannehill's riding the pine for a year.
Palmer's dead money, if he's out after 2017 = $6.6m
Rivers' dead money, if he's out after 2017 = $12m
Tannehill's dead money, if he's out after 2017 = $4.6m
Reports on Bridgewater vary, ranging from career threatening to just out for the season. Reports on Sam Bradford are that he's not very good, and he is very brittle.