Why would they do it?
You are the Cavs, you have been to 3 straight NBA Finals and your second best player has demanded a trade because he does not want to play with your best player, who everyone has been saying for months is going to leave your team after this season. Your second best player is young and is under contract for 3 more years, with an opt out after 2. You have no good young players on your roster and your draft pick is not going to be substantial. You have to think outside the box. You can trade good second best player for lots of stuff to put with your best player, but your best player can leave in a year. By acquiring Porzingis, you have a great young player to pair up with Kyrie Irving. They can grow together. Porzingis will be happy to get as many shots as Lebron or Kyrie were getting last year. He gets out of New York and you as the Cavs now have two pieces under 25 that you can build around.
Why wouldn't they do it?
Lebron James has a no trade clause. He can decide if and when he leaves. He would have to approve of any trade and he might not want to go to New York. However, this could be his chance to leave Cleveland again without dealing with all of the traitor stuff. He could put it all on the organization saying that they chose Kyrie over me. In terms of salary, you are going to have to take back Courtney Lee and Joakim Noah, two players under contract for 3 more years. They are making a combined 28 million, which will eat a huge hole in your cap. Kyrie might not commit to your team long term. He might say to the Cavs that he is going to leave after 2 years no matter what you do. Also, let's not forget the fans. They will hate Dan Gilbert for this move. They might also hate Kyrie Irving. This could be Kobe and Shaq all over again. Yeah, the Cavs might be bad for a year or two, but Kyrie and Porzingis could figure it out.
Let's go to the Knicks:
Why would they do it?
The Knicks have been terrible for many many years now. They have not been relevant to the league in a long time and have not been contenders in longer than that. While this move is sacrificing your future, you are going to have a hell of a 2017-18 season. Your fan base will be so excited that they get to see Lebron 41 nights a year. In the Eastern Conference, they immediately become contenders. Lebron gives you positive vibes for the whole season. This possible one year rental is totally worth it in my book. Keeping Porzingis is only going to get messier and there is no guarantee he re-signs with your team. By agreeing to give him up you can bring in the best player in the league and immediately flip the fate of your franchise for one season. This might also get Melo excited to stay in New York and play with one of his great friends. Also, you get out of the Noah and Lee contracts that were going to haunt you for the next three years. Even if Lebron leaves you will be financially set with a huge amount of money to go after free agents.
Why wouldn't they do it?
The Knicks are not willing to sacrifice their future and refuse to give up Porzingis. Carmelo Anthony hates your front office and organization so much that he refuses to let go of his demand for a trade. Also, maybe you do not want Carmelo on your team any more. The last reason would be that you are not willing to risk Lebron leaving after one year for nothing.