Boston might have the best package, but CLE doesn't want to help Boston get better. Mostly b/c they will play each other in the ECF. If Lebron loses to Boston in the playoffs, he might be more likely to change locales (though I do think Lebron will retire as a Cab.)
I think Denver is a good spot for Kyrie. Any type of combo including Garry Harris/Mudiay/Jamal Murray/Faried/Barton/Hernangomez/Trey Lyles + picks could get this done. Cleveland improves their bench and overall roster, and won't have to spend a ton, allowing them to somehow pull of a trade/signing for another top player willing to play in CLE with Lebron.
I think Dallas makes sense. Dennis Smith and non-Dirk change. Cuban clearly doesn't want to lose games and bottom out.
Sacramento, Minnesota, SAS, and LAC all seem like good fits for Kyrie.
They are also all in the West. If I was running CLE, I wouldn't trade Irving to the East, b/c I don't think we have seen his top gear yet. Hate to see the guy get to the next level against you in the playoffs.
The whole point of the rumor is that the Cavs aren't going to make a trade with keeping LeBron in mind. If LeBron told the Cavs, "yeah, I'm in if you make X move", then that would be the trade that gets made. But LeBron wants to keep his options open, reportedly. That's perfectly reasonable for him, but accordingly the Cavs are going to make the trade they feel gives them the most value long-term, since LeBron might leave no matter what. If that's with the Celtics, so be it. Their goal is to not have a complete teardown like the last time LeBron left, but to continue to field a competive team, hopefully infused with some younger talent.
Now, as many have said, ultimately a deal with the Celtics doesn't seem to make much sense. Even if one concedes that Irving is an upgrade over IT (which is a highly debatable point), he's not much of an upgrade. Adding in a recent top 3 pick like Brown or Tatum is a huge addition in price. And even adding one of those with IT still isn't enough in outgoing salary, so you'd need to add in still another player, who would either be a rotation-level vet or an additional prospect on a rookie deal (or even two such players, depending how much said players make and whether or not Kyrie waived his trade kicker). The alternative is pairing IT and Kyrie, but that's a gamble in its own right.
If a deal between the Celtics and Cavs doesn't happen, I doubt it will be because the teams are afraid to trade with one another. It will be due to the fact that the Celtics can't match Kyrie's salary in a way that gives the Cavs what they're looking for without being a massive overpay.