I think watching T.Y. Hilton is an exercise in seeing what a good athlete with sub-par skills can do with the best ball-tosser in the free world as his QB.  I don't think he runs routes, makes plays, or even catches particularly well.  But he's fast and has been part of the system for a couple years.  And all you need is a tiny bit of separation for a guy like Luck to put the ball in your hands as you streak through a very small window most QB's couldn't make use of.
I think Dorsett is a better WR than T.Y.    
 
T.Y. will certainly start the year as a much, much bigger part of the passing O.  But I think by the end of the year, it could be painfully obvious that he's no better than the third-best WR on that roster.  He may still end the year with the best numbers, but the writing will be on the wall by week ten or so.  And if he bolts in FA, I think a lot of people may be disappointed to see how mediocre he is with pretty much any other QB at the helm.