Honestly if we don't sign Greg Monroe or DeAndre Jordan, Melo is probably going to ask for a trade, and honestly, I'd have no problems with him gone. Today's NBA requires ball movement and team play. Players who only know how to iso are not the ones winning titles or even going to the Finals. Carmelo has never been a winner in his entire career and there's no reason to think that he'll suddenly stop being a ball movement-stifling player. Trade him to a team that will take Jose Calderon's contract, get at least 2 first-round picks and two young players in return, and rebuild the team around Porzingis. In the meantime, sign Jeremy Lin to keep the franchise interesting. A team built around Lin has the same probability of going to the Finals as a team built around Carmelo - a 0% chance. Finding a trade partner will be difficult but I'm sure as the season unfolds that a team will want Carmelo's scoring.
It took the Warriors 6 years after Stephen Curry was drafted to get to the NBA Finals. If Kristaps Porzingis is anywhere near as good as Phil Jackson was claiming him to be, the Knicks could be a contending team in as little as 3 years. You'll have a better shot of luring players with young, attractive talent like Porzingis (if he's good, which he should be as long as he puts on some muscle) than you will with Iso-Melo