Okay, lets take this point by point.
Its true that a lot of guys put Jaylen Brown at number three after he had work outs with the Celtics n the weeks before the draft and rumors started circulating that they were interested in him. But there were cterianly mock drafts that had him going to the Celtics even before these rumors spread. SI for instance had Brown as the 3rd pick a full month before the NBA draft and before they brought him in for workouts. So the idea that his mock draft ratings were based entirely on where people thought he would go are false. This is further disputed by the fact that there were mock drafts that had him going right after us to the Suns. To Nickagneta, I refer you to mock draft from both Fox Sports One and Forbes who at one point or another in the month before the draft had him going to the Suns. If you think about it this makes sense, the Suns the very next year took Josh Jackson. An athletic swingman who was strong defensively but cant shoot (sound familiar?).
Now, mock drafts are often bases on projections, but this idea cuts both ways. You can say that mocks had Brown at 3rd because of the Celtics rumor, but a lot of mocks didn't have him at third because he didn't fit our perceived needs at the time. He wasn't a shooter, and he wasn't the guy the Bulls were rumored to be interested in. A lot of that Kris Dunn hype was because we were gonna flip him for Jimmy Butler. In other words, maybe Brown was a worthy pick at 3 but wasn't mocked there because of perceived need. The very fact that so many of these mock drafts were trying to project 3-8 based on teem needs tells you how closely the 3-8 prospects in the draft were ranked.
Now I'm willing to concede that in general more mocks had him going 7 or 8 than 3-6. A 2:1 ratio in that regard sounds about right. But thats far from a consensus. And as I've already pointed out alot of msart people like Kevin o connor and Nate Duncan has him ranked pretty high. Perhaps the best argument that Jaylen was in play much higher than 7th or 8th is the fact that Dany took him at 3rd. If Ainge could have moved back to 7th and gotten Jaylen, who he obviosuly thought was the best pick, don't you think he would have? Even if only for an extra 2nd round pick or late first? Obvisuouly he would have, he did it with Tatum. the fact that he didn't is a pretty good indicator he thought Brown would be gone by then. Probably to Pheonix at 4.
Now let me give you guys a history lesson. Before that college season Jaylen Brown was rated almost universally as a top 3 pick. He was the 2nd rated recruit in his class our of high school. A bad season at Cal culminating in a Kevin Pelton piece that has Brown projecting terribly in statistical models moving forward tanked his position in mock drafts. But then as people did more in depth analysis he started recovering a bit. People pointed at that Cal was a terrible fit, he was asked to basically play PG which he wasn't equipped to do on a team with a bad coach and no spacing leading to terrible efficiency. With NBA spacing he would probably fit better. In addition although he shot bad at Cal from 3, he was a pretty decent shooter from distance in high school. Then he started going to workouts, and not surprisingly he is the exact type of player who looks awesome in an empty gym and his stock recovered yet further. Brown became a test case for analytics vs traditional scouting and upside projection. Hence the fact that he was all over the place in mocks.
Anyway, my point in all this is Brown wasn't a reach. If you went ahead and asked all these guys writing these mocks who had him 7 or 8 whether they'd be surprised if he went 3, they would say no. And thats my point. That was the prevailing opinion at the time, 3-8 was fluid and although Brown has his warts so did everyone else in that range. We shouldn't be sitting here giving Ainge credit for seeing something nobody else did when numerous people had that as the pick. Hell, I had him number 2 on my board and I am far from a draft expert! That being said Ainge sill made a hell of a pick, having #3 in a supposed 2 player draft and coming anyway with arguably the 2nd best guy is impressive.