The problem isnt a single decicion like wether we should've traded Barnes or not. The problem is the bigger picture, the process or the strategy what ever you want to call it.
Its the horrible process that causes a lot of these mistakes that generate these hidsight discussions. like
@The_Jamal said, the process/strategy caused that decision, same as trading away Haliburton ect. If you want to win now and maybe get to the 8th seed/play in, you will not trade Barnes for a 1st and a prospect when he still has some trade value as an aging vet getting close to needing extension as unrestricted fa, you wont trade Holmes when hes playing a career year, you will trade a promising young guy that loves being here and you have basically 6 more years of his rights for a player thats basically in his prime and enters UFA and needs a huge contract. You also spend 50million a year on vets right after trading away your franchise player (aka being in a perfect position to rebuild but of course not because win now). The list goes on and on for multiple years back.
We've not been the worst franchise in the league because of some single trade desicion. When the method, the way of doing things, the strategy ect is continuously bad it will lead to bad decicions and bad results. Thats exactly why this franchise has been the worst for the past 10+ years of time frame and it most likely wont change as long as the process stays bad.