Of course the other way of looking at it was that the Celtics gave up their big man in big Al, and went on to win a championship. The Celtic fans wern't happy when it happened. Both sides took a gamble and one side won. The difference was, KG was part of a bigger plan, and big Al wasn't.
The Celtics gave up a 15-10 kid with limited defensive potential to get a superstar.
Any trade for Cousins would be the same thing.
Rebuilds are crap when you already have a superstar in the fold. Then all you are doing is wasting time going out hoping/praying you can find again what you just gave away. And hey, it only took 30 years to find Cousins, so happens all the time and all that.
It took Philly 5 years to get another star in town to replace Charles, and it was A.I. I wonder how weenie Kings fans would have reacted to that guy's antics? or Sir Charles' for that matter. In any case, A.I. never got them over the top either, and since he's left they have never recovered and have been playing gimmick games for years.
It took Minny 3 years after KG to eventually get Love, on a lucky draft day trade even. Love was never able to get them over the top either, then they dumped him and have gotten worse. Maybe in a couple of years they can tar and feather Wiggins and KAT too and start over again.
The Hornets (now Pels) traded Mourning and even putatively got back a lesser star in Rice. Good for a couple of harmless playoff runs, then 5 years in the wilderness, and a franchise relocation before CP3 arrived.
The Magic had to wait 4-5 years to try to TMac/Hill gambit after they lost Shaq. That didn't get far, and a few years later they were rebuilding again around Dwight. They lose Dwight, obviously declining, and they still have never been as good while searching for a new "the man".
For all the hoopla about Denver thriving without Melo, that lasted about a year and a half. Its distant memory now as they struggle along without a star and hoping to just get lucky with some kid or the other. Mudiay will be there new hoe. Maybe ready by 2018.
The Wizards dumped Webber, before he was truly a superstar, and waddled on for years until lucking into Arenas. Then that ended uin diaster and finally resulted in Wall some dozen years after they dumped Webb.
The Bulls had a 10 year hangover between Jordan and Rose. And of course the Rose era lasted about 2 years when it finally came.
If you have a star and lose him, it can take years, half a decade or more in many cases, before you find another guy who truly is. If you were to trade Cuz and get back anything less than KAT, Porzingis, or the #1, this whole board, if it even survived, would likely be a whole lot greyer before they saw his like again.