People are claiming Brock will be leaving, because there are signs that suggest he won't be in the anime as a main character anymore. For one, all this season Brock earned little to no attention (you'd think the writer's have realized his story is done with). Secondly, after Ash finished in the Sinnoh League, we had Brock talking in his mind, and I believe when it was translated, he was saying something along the lines of, "Now, that Dawn and Ash has been going towards their dreams, what about him?" In addition to that, he's been said to have a new dream - turns out it's to be a Pokémon Doctor. Thirdly, if you watched the latest Japanese episode, you'd see that that episode was focused on Brock for a reason - so people will understand why he's leaving Ash later on to do his own thing now. Not to forget, Brock wasn't even on the beta art scan that was leaked a few months ago, while Ash, Iris, and even Team Rocket and a strage old man was there. Lastly, in DP191's preview, they put a significant look onto Ash parting with Brock. You can't blame people for not saying Brock is leaving for sure, because there are a lot of clues that lead behind it. It's called a foreshadowing, if you must know.
I'm not saying it's confirmed, but at this point, a viewer is to realize Brock really isn't going to be traveling with Ash and Co. anymore. With Dento being a Gym Leader and deciding to travel with Ash, what's the purpose of having Brock anyway? So, even though it's not confirmed, one can pretty much assume it's going to happen.
You can deny it until you have your "confirmation," but either way, in the end people will be coming to you and saying, "We told you so." =P
