I don't think the threat of a hard cap is a good argument against acquiring talent.
For one thing, it's far from a certainty that a literal hard cap will get inserted into the next CBA. Sure, it's good to act in a precautionary manner when the future is a bit uncertain, but use that caution wisely. Don't throw 4/$50M at Erick Dampier, even if you can afford it now. But as we've seen, nobody is being very shy about giving out 6/$120M contracts to the right players even with an uncertain future for the CBA. Carmelo is the kind of guy that you should give the money to, and then cross the financial bridge when (and if) you get to it.
For another thing, if it turns out that a hard cap does get installed, then it's going to hit everybody, not just us. Maybe the hard cap means that you can only get two max-type contracts on a team. If so, then when the time comes we max Tyreke, then the next year we max Cousins and let a 30-year-old Carmelo walk. And if Carmelo's contract is too big to max out Tyreke (which seems unlikely), then we trade Carmelo a year early. It's not like somebody out there won't want him. In any case, what do we lose? We get Carmelo for 3-4 years and see what kind of a shot we can get at a title towards the end of that time frame, then we revert to a 2-max player team like everybody else. If we don't go after Carmelo, at that point we're in the same position (an Evans/Cousins team) but we spent the four years to get there without that all-star SF.
We can't forever sit back and try to stockpile young talent, and wait for that young talent to develop. At some point, we have to say carpe diem and go for it. And if we could actually get an extended Carmelo without shipping off Evans or Cousins, I say it's carpe diem time. Make the team very very good right now, and deal with the possible consequences of the CBA later. If nothing else, imagine what turning the Kings into an immediate contender would do for the chances of financing a new arena.