IT was better than Kyrie last year. That's not really debatable. More points in fewer shots, better efficiency, more assists, etc.
Whether IT ever reaches those heights again, or whether Kyrie ever does, is a question mark. What isn't is that IT had a better year last season than Kyrie has had to date.
Once again, you cannot compare the two seasons given that Kyrie was forced to squeeze his game into being what Lebron wanted it to be (as does everybody who plays with Lebron). How many star players have shared the court with Lebron in the past? Chris Bosh, Dwyane Wade, Kevin Love? How many of those guys have had to restrict / compromise their game to some degree when they joined up with Lebron? All of them. Chris Bosh and Kevin Love even publically commented on how difficult it is/was playing with Lebron, because of how dramatically they had to change their games (and how much less freedom they had).
In the meantime IT was given the keys to the Celtics and ultimately had completely freedom to play the game freely however he wanted to, with the rest of the team pretty much falling in line around him.
So you really CANNOT compare those two seasons in terms of outright products. If IT is able to out-produce Kyrie this season (when their situations are reversed) then I will give it to you and acknowledge that your point is valid. But I'm willing to bet that that doesn't happen. I'm wiling to bet that Isaiah's numbers are going to go down significantly, while Kyrie's will go up significantly - and the Kyrie will have a (significantly) more productive season.
Lets see how that pans out, shall we?