Simple question, is Bagley better than G. Williams. If the answer is yes, then the question is how much better.
FYI, Randle wasn't traded. Carter was in the Vucevic trade. Parker blew out his knee multiple times (and wasn't actually traded and then signed a 2 year 40 million contract back when that was a lot more money). And none of those guys really produced like Bagley has (well Parker did until the knees gave out). Though Morris did land a 1st round pick (ended up 13th), a young player, and a vet. He was 26 and in his 5th year, so not exactly the same as a 22 year old big man. The reality is most young players don't become available during their rookie contracts because most aren't drafted to wholly incompetent organizations and often the ones that are traded are pieces to a larger trade where the team trading them is getting a "star", think trades like Carter, Poeltl, Ball + Ingram, SGA, Dennis Smith, Thon Maker, etc. Most teams wouldn't dare trade a top 2 pick still on his rookie contract who is averaging 14/7 in his career, but the Kings aren't most teams.
I just don't rate draft picks outside of the lottery all that much. They just don't move the needle for me. And I fully expect Boston's pick this summer to be around 22 and if there was a future pick included it would be lottery protected. I just don't think that is all that much to give up to get a clear upgrade from Grant to Bagley (and hernangomez is just going to be salary filler in some trade at some point - I mean he didn't even play in game 1 with the C's down 2 rotation players).
I included many of those as comparables, even if some didn't end up being traded. In many cases there was significant reporting around the possibility of trading them and what packages were out there. There's a reason these guys don't come available - because generally they flame out. If a guy can't make a mark after several years on his first team, it's not often they become all-stars, or even starters.
I'm one of the biggest fans of Bagley's game out there. I think he is a genuine talent, but I'm arguing your valuation is way too high. Distressed assets like him are simply not traded for as much value as you are suggesting. It does not happen. If you come up with some examples, I'm willing to listen to them, but I can't think of a single high lottery pick who played for his rookie lottery team for multiple disappointing seasons, couldn't earn a starting gig toward the end of his rookie contract, and then was traded for two first round picks, one role player on a rookie contract that has made good contributions on a winning playoff team, and one veteran role/bench player.
Most lottery picks who can't earn a starting position at the end of their rookie contract flame out of the league, or become nothing more than bench players, which is why it's hard to value a guy like Bagley as if he is as valuable as you are saying.