If we're playing this game we can't assume that we know we are only getting one playoff run with Kyrie. You have to assume the parties in the trade believe he may be available for two playoff runs.
With that calculus in mind, considering the rest of the talent the Celts had/have on the roster, you're basically asking if two potential Finals runs is worth a top 10 draft pick. I say yes, especially given how much young talent the Celts already have and how many draft picks the Celts have in the pipeline even after the trade.
Except the hypothetical presented suggests that you *do* know ahead of time, just like you know it’s the #8 pick and that Kyrie is leaving.
I don't see the point of the hypothetical. The deal has already been made so why ask questions based on info that wasn't available at the time the deal was made?
I agree that the fact that kyrie is leaving and that the pick is number eight are also things the parties didn't know at the time of the deal. But I think the value of the pick is something the parties can reasonably estimate.
As for Kyrie leaving, I think it's interesting to entertain the idea of whether the trade would make sense if the parties knew going in that he would likely leave. Teams consider deals like that all the time.
Anyway, forced to answer based on the idea that the parties know that Kyrie will only be available for one playoff run, I'm inclined to still say yes to the deal because the Celts are that close to making a run to the finals and perhaps even winning a title. It's a much closer thing, though.
Luckily, we don't have to assume that Kyrie is leaving because there's a good chance he isnt going to. And if he does we shouldn't judge the trade as if that was a foregone conclusion.