If you are rebuilding you don't hang onto a player that won't let you be truly awful.
To rebuild you want to rebuild around a star. Rondo IS a star. What's the point of tanking for the lottery? For the off-chance we get a better player, which is by no means guaranteed? Have you not noticed that the vast vast majority of lottery picks don't turn a franchise into an instant contender like OKC?
It IS possible to rebuild through the draft, but it isn't by any means a guarantee. Especially with what is projected to be a weak draft this season. We already have a star, it makes no sense to get rid of him for the purposes of tanking. We either get rid of him for a real star, or build a team around him.
Cousins and Evans for Rondo (as an example) is a pretty solid rebuilding trade. You get 2 younger and just as talented players for 1 older injury prone, but better player.
Why are Cousins and Evans any better for rebuilding than Rondo?
That is what you should do unless your team is one of the top contenders, and for the record, OKC actually traded its third best player, for multiple lesser parts and is probably just as good as they would have been with Harden and have much greater flexibility going forward.
To win in the NBA, you need a star player. Nobody questions this. We have a star player. Granted, he is not Lebron, but he has proven on multiple occasions that he is a star, and by virtually every statistical method of measuring basketball he ranks extremely high.
You are supposed to build around a star. What's the point of getting rid of a proven star for the CHANCE of getting another star in the next couple of seasons? All it does is make us a bad team for a couple of years for the off-chance we get as lucky in the draft as OKC did. What happens when we don't get a star in the draft? Then we are just bad forever.
So we either keep the star we have and try to build a contender around him, or try to get another star.
I will give you that trading Rondo for pieces could potentially help us land a star in a trade down the line, and Danny is smart enough to pull something like that off.