Ting has a contract MVNO agreement. Unless Sprint goes out of business entirely, or breaks their contract (very unlikely), nothing will change for Ting at all.
Ting might have to make folks buy new devices (if T-Mobile can succeed in finally shutting off Sprint's legacy 1X and EVDO CDMA stuff), but nothing should happen to Ting that would prevent them from operating.
They're awesome, and T-Mobile's been slowly adopting pieces of Ting's pricing/openness policies.