One thing we all need to keep in perspective with Lynch is he's only 26. That being said, it's realistic to say that he is in his prime until he's 29 or 30, barring any health conditions. Running backs seem to get thicker/slower about that age, to the point they become less effective.
Turbin is a nice upgrade from Washington and Forsett. Forsett was too indecisive and while quick, didn't have a real burst. It seems he's doing a bit better than he did here in Houston, but he is certainly a backup RB. Washington is a solid football player. He can do a lot, but I have never seen him as an every down back - although if pressed, I'm sure he'd do just fine.
Turbin, I see him as a RBOTF potential - he needs to thicken up his base, but the speed, quickness, and burst are all there. I love Lynch - and I think by next season it wouldn't surprise me if we all assessed Turbin as a high-value backup who is capable of being the full time starter.