Yes and no, a lock-on within the visual range of the seeker is not going to give an aircraft too much time to outrun it, aircraft at low altitudes aren't going to be able to go supersonic very well anyhow, even if it's a fighter.

Also modern MANPADs have sustained supersonic speed which is why they are only good for 4-5KM, but within that they'll splash you at max speed. A SA-18 traveling at 800M/S+ has only a range of 5000M so there isn't any "coasting" this isn't a movie they fly for 5-6 seconds and either hit something or drop like a rock.