While the enormous turnover might seem alarming at first for a team that just finished first in their conference, it kind of makes sense to me for a couple reasons.
1. We were faux contenders last season. My view regarding trade rumors last season has always been: "Don't trade away any prime future assets in a deal for a Butler/George. Danny's original plan was to be contenders by 2020/21-ish, so let's enjoy this ride that came along by surprise, but not jeopardize the original plan." Well we did trade away a prime asset (BKN pick) which I wasn't pleased about, but if Danny thinks that acquiring a 25 year old Kyrie with that asset improves our chance at contending in 2020/21, then that is the only way I'm OK with the deal. I had been enjoying the ride, but really it would have taken an absurd amount of luck (injury to key opposing players) for that team last season to win a title.
2. The unexpected "contention" last season was nice, but also surged the value of our players coming up for new contracts. It caused a huge increase in the values of Olynyk, Isaiah, and Avery. From a purely rational perspective, it's wiser to let players walk or deal them and get value for them when their value is at its peak like the Patriots do. And the Celtics team's success last season has pretty much peaked all of their values.