If the salary cap era started in 1985 let's forget the 80s Celtics and Lakers. The key players were acquired before.
The title clearly states dynasties not just a single championship.
So how many dynasties have there been since say the late 80s? The Pistons, Bulls, Spurs, Lakers, Heat, Warriors. You could argue maybe a few more like the Cavs and C's even being in the finals twice but I left them out.
The Pistons top 2 players were Dumars and Thomas both homegrown as were many of their players. Same for Pippen and Jordan. Kobe was drafted and Shaq a FA. The Spurs major pieces were drafted. Heat is different with only Wade, but that was almost once in a lifetime thing. 3 out of the 4 Warriors players are homegrown.
What the OP makes sense. Generally if you draft multiple stars at a young age you can have sustained success with a possible dynasty.
Trading for stars as they get older most likely you will have short lived success, a 1 championship and done kind of thing. I can see what he is trying to say.
The Lakers did not draft Kobe. he was acquired like a month after the draft well into free agency season. He never played a game for the Hornets, but he wasn't a draft day trade, he was acquired via an actual trade. Pretty similar to how the Wolves acquired Wiggins.
I think it still falls under his guidelines as a drafted guy. Basically what I think the OP is trying to say is trading for an established star with an expensive salary it's harder to build a dynasty with sustained success. I'm fine with that guideline. Pistons, Bulls, Spurs, and Warriors (aside from Durant) are dynasties built that way.
There is no practical difference between trading for or signing in free agency. I mean take the Heat in order to have enough cap room, they started selling off players and sometimes even included 1st's and thus the Heat gave up a lot of valuable assets in order to acquire James and Bosh through free agency.
The Rockets would have had to do the same thing had they not worked out the trade with the Clippers.
The Warriors had to dump Bogut and Barnes and let Speights and Ezeli go in free agency to create the room to sign Durant. They likely will have the same issue this summer as well. They were able to sign some competent replacements for the minimum or exceptions, but there was certainly no guarantee of that and they might not replace someone like Iguodala if he leaves this summer.
Even Boston is going to have let some valuable starter/rotation players go to sign a max free agent this summer.
Sure a pure trade probably costs more, but there are real costs to creating max cap room to sign free agents.