At this point does it really matter? I know we want to feel justified for losing him for nothing because we gave up assets for him. I'm just enjoying Celtic Basketball again for the first time in a few years and for me that alone is enough to justify using Kyrie as the reason the Celtics did not live up to expectations last year. that and the fact that I watch every game and it's easy to see how Uncle Drew dominated the ball, slowed down the offense and was a horribly poor leader. This year it's different. It's fun, guys are active and look happy. This alone should prove the narrative that Kyrie was a cog in the wheel.
Since we were able to sign Kemba at the same time Kyrie left, I don't think people really care anymore that we gave up assets to get him. It more seems that we gave up assets to trade for Kemba. We've been lucky to have All-NBA PGs these past several years with IT/Kyrie/Kemba and with IT having his injury issues, Danny made a bold move that basically paid off in the form of Kyrie. The way I see it, Al wanted a different role and more money on a better team. It was still possible to keep Al even after signing Kemba (although it would have been more complicated).
Anyway, you are basically right - because last year was such a mess and because he chose to leave us after verbally committing to us, many people are just happy to hear things might not be going so well. It is a little petty, but not unlike a personal relationship that might have ended badly. You might not want that person dead, but you don't want massive success for that person either.