All I know is:
2. But on top of #1 we actually ADDED that player to our own roster. Sure we lost our own best player, but that player is hurt, we wouldn't have had him the entire season and we don't know if he would be likely to get injured again before or during the playoffs.
If the first bolded part is true, then this trade is a huge win. But we don't really know that it is true. How confident one is of this prediction is the crux of whether one thinks this trade is a big win or not.
The second part in bold is true for every player. Are folks forgetting that Kyrie missed a big chunk of the playoffs not that long ago? Injuries can happen to any player, young or old.
The #3 & #4 parts of your comment don't really have anything to do with this particular trade.