Ian Begley who covers the Knicks just asked Kyrie if his mindset has changed regarding resigning with Boston, Kyrie said "Ask me July 1st", he said he's going to do what's best for his career.
The scenario you laid out is exactly how Danny Ainge would like it to play out, they would move forward with a "Big Three" of Kyrie, AD, and Tatum, and build around them, but it's really all up to Kyrie. I know Kyrie and AD are friends and have spoken in the past about playing together, and like you said, all he has to do is tell Danny he'll stay in Boston, then once AD knows that, there's a good chance he would sign long term in Boston as well, but from everything I read and hear, Kyrie might want to jump ship. Why he wouldn't want to stay in Boston is something I don't get, unless he's hell bent on playing in NY (he's from Jersey), if that's the case Boston is screwed since there's no way AD signs long term without a commitment from Kyrie. Kyrie really holds all the power right now, and maybe he is planning on staying, he said before the season he would resign with Boston, he even did that commercial playing one on one in the Garden against his dad, talking about wanting his number to hang in the rafters someday, but now he wants to do what's best for his career? What's best for his career is playing for the most storied franchise in the history of the league, playing potentially with AD, playing for an outstanding coach, and one of the best GM's in the business. I just wish he would come out and say he's staying, then the odds of AD staying become better and Danny doesn't have to worry as much about a one year rental. I probably shouldn't let this stuff bother me, but I've been a die hard fan since the Larry Bird days, and the stress Kyrie is causing me is driving me crazy, just praying everything works out and things go exactly how Danny wants them to.