Well, for once, Keldon and Collins are already yours and won't cost you 3 or 4 unprotected first round picks.
Also, getting the wrong star player absolutely sets your team back years. Plenty of examples to go around: Westbrook in Houston, Harden in Philadelphia, Kyrie and Harden in Brooklyn, Beal in Phoenix, Murray in Atlanta, etc.
It's not as easy as "let's swap this two mediocre players for a flawed star since they are making the same money". When you make a commitment for a player of this caliber, you are making a multi-year commitement, if said player isn't the right choice, you are stuck with him for a while and getting out of that situation might be just as costly as getting into it in the first place.