Great players don't get traded on the first two years of their deals. And deals are now all 4 years (with player option after 3) or 5 years (with a player option after 4).
Any player we trade for is going to be either a FA after a single season or best case scenario you get them for two. If you're not willing to trade for a rental player or a short term deal you're just not willing to trade for great talent.
TP, very underappreciated point. Teams aren't just willingly selling away top 15-20 players unless they absolutely have to.
That doesn't make them worth paying more for, though. If you aren't confident that you can retain the player past next summer, it's a rental.
Westbrook in particular scares me that he would not be willing to re-sign in Boston because:
a) He's clearly going to leave OKC -- so that means that Bird Rights' contractual advantage is not going to be enough to convince him to stay. So trading for him now would give Boston no particular advantage due to Bird Rights. He is perfectly willing to sign a non-Bird FA max contract with a different team in order to exercise his finally-won ability to decide where he wants to play.
b) He's from Long Beach, CA.
c) He's got pretty well known business and personal interests in the fashion industry and celebrity pop culture.
d) He just bought a 4.65M mansion in Beverly Hills (from some dude who is on one of the reality shows (Kardashians?).
e) The Lakers will have enough cap space to sign him to a max contract plus an attractive, "Come back to your home town and rebuild the next great Laker dynasty with all our young talent, Russ!", sales pitch.
d) The Clippers almost certainly could move one of their big assets (CP3) to clear cap space to sign him.
What about any of that would give you confidence that he would stay in Boston?
All signs to me indicate he will most likely end up going to one of the LA teams.
Sure, any star we might trade for is likely to only have 1 or 2 years remaining on their deal. But I don't think all one-year-rentals are created equally.
For example, if we traded for Gordon Hayward, I'd expect that the probability that he would sign long term with the Celtics to be extremely high. That said, that doesn't mean I'd want to trade very much for Hayward, either. Because if we think we have a very high probability of signing a player to sign here long term, why not just wait and sign him next year in free agency, and not give up significant assets?
Ultimately it comes down to price. I'd be willing to give up a little for Hayward to get him here early, but definitely not a lot because he's not going to win us Banner 18 this year.
And I'd be willing to give up even less for Westbrook to get him here early because I don't have faith that he will resign and, again, he's not going to win us Banner 18 this year.