First of all, please don't compare Metcalf and Boykin. Metcalf showed more in a single season than Boykin ever has.
Second, I think comparing Brown and Metcalf is deeply flawed. They have wildly different wing spans, verticals reach, and should be used differently. The fact that there is some overlap in the "they both run real fast" department should not indicate that they are even close to the same guy.
I think its no shock that big bodied Mark Andrews was by far Lamar's favorite and most reliable target, while Hollywood had trouble getting the ball in space or thriving in the short and medium game. You need to be able to thread needles and have great anticipation of direction changes to get balls to small receivers in traffic, and I don't think Lamar is at his best when he's doing that. A bigger receiver with a similar skillset but a wider catch radius is likely to be a much more attractive target.
You're right, they're not the same guy, and I pointed out their differences. But their strengths are very similar.
While Metcalf is a great receiver I think you are underselling the extent to which he is a one trick pony. He almost never runs anything other than go routes and underneath routes in the flat to take advantage of soft coverage. I am not certain that Metcalf can consistently get separation on mid range routes. He's literally never been asked to do that. Brown can run every route in the book and get separation. Yes I know that he has T-Rex arms, but he at least gets open. If you want someone with that kind of versatility then you absolutely don't want Metcalf.
You can maybe make an argument that since we have Andrews to act as Lamar's Tyler Lockett, someone like Metcalf would do a lot to keep defenses honest. The problem with this is that Lamar doesn't really have the best arm on deep balls. Probably not as good as Russell Wilson's. Maybe having someone like Metcalf scooping up everything within a 10 foot radius helps, but I am not convinced that it's enough. Especially considering we dont throw the ball that much.