There's no way he's going to play PF full time, especially at his age. And there's no way the Knicks want to trade Melo and ALSO lose their pick. If anything, they want to add picks, not players.
Also, what's the point if the lineup you end up with is starting Smart and Jaylen? I love Jaylen, but he shouldn't be starting for a team that wants to win now. And Smart is so much worse on offense compared to Bradley.
I don't understand why people want to go after guys like Melo or Millsap. It doesn't make us better than the Cavs and they have what, one year, two years left at most? Carmelo is 33 years old! We're trying to beat LeBron and we sign a guy who is worse AND older, yay.
Finally, signing or trading for these old guys basically means that cap space is totally gone, OK? Because these guys are old they are untradeable. We'll be stuck with them until their contract expires and chances are we'll still be over the cap because the kids will need to be extended. So basically we lose all flexibility and we lose that space for nothing.
The Celtics are not going to waste their last season with cap space on old players unless their name rhymes with Jebron Lames or whatever. They're going to spend it on a young star who will still be in their prime throughout most or all of the deal (basically Hayward), or they will spend it on smart extensions/tradeable/dumpable free agents so they can protect their flexibility to trade for a star in the future.