I don't see Kyrie resigning with Boston when his deal is up though

That's two seasons from now. They had to deal with IT now. They were seeking to trade him anyway to avoid to pay a max deal or him leaving and not having anything in return.
Outside the draft pick. This doesn't move the needle with the Cavs AT ALL. LeBron is still leaving, IT is a RENTAL and what is left the following year? This isn't going to help them beat the Warriors which is the main objective.
Guaranteed in two years, the CELTICS are finals bound barring injury. Ainge looking at the bigger picture, and still able to keep cap money and have more draft picks on deck.