When May first arrived people were stunned, but after the shock of Misty being gone eventually settled in, May became a well-received character. Same for Dawn. It's like I said in my opening post, the writers do a much better job introducing new female characters than they do new male characters.
It's worth noting that a significant number of fans left the fandom as soon as Misty was gone. There were many people who thought the show jumped the shark (Sharpedo?) when Brock left, but he came back soon enough that it didn't have too big of an impact. There still are plenty of people who feel the show jumped the shark (Garchomp?) when Misty left, though.
But it's hard to find them because, well, they're not in the fandom anymore, so it's not like you can see them posting on the forums or voting in polls. I've found most of the ones I've run into by bringing it up on different livejournal communities and luring people in conversations about the series by holding Pokémon plushies at anime conventions.
I also find it hard pressed to believe that if they took out Brock now, that anyone would care on the same level as when Misty or May were removed.
Maybe not. It's impossible to deny that he doesn't get the same level of appreciation as other characters. But I can say with some confidence that plenty of people would care. Especially when one considers that they've really been giving him some well-deserved attention finally.
You can go ahead and call me biased on this next part, but: Brock is an excellent character, and I would argue that he is one of the most, if not the most, dynamic and multi-faceted characters in the series. Seriously, I've tried expanding the articles of the other characters to the same level that I've expanded Brock's article on Bulbapedia, and... it's difficult, to say the least. But in DP, I think we're seeing the best of his character more than ever, as both a comic relief and a serious character - though it's still restricted by the fact that he's there to support Ash and Dawn, and I think a lot of Brock's fans understand and respect this fact. (Even if it would be awesome if he had a more active role in the plot.)
Tracey and Max were both so-so. Yes, both got shafted by not getting the focus and development they deserved. But Brock was filling the same spot as them since the fifth episode, and even co-filling it with Max through AG and BF when there was even less room in that role to share. He was shafted, too, and unless they finally reach the point that they can adequately juggle three main plots, any character who could replace him would be shafted, too. And despite being in such a difficult role to fill, he's got more fans than both of them combined (even if that's not saying much) and a well-developed character whose antics are only beat by those of Team Rocket.
And that is, in my opinion, why he's still here.