To be fair if writers wanted they could create reason to have him join group again.For example his goal is to become doctor,well if he decides to become travelling doctor he could conveniently join Ash on his journey travelling through same region to learn about new things.Nope. He has no reason to come back as a member of the permanent crew, if that's what you mean.
Thats matter of opinion in reality.I met many people who still find Brock entertaining eing sads to see him go.What's with fans of the Pokemon anime always complaining about long-time characters, that have been losing their flavor, finally leaving the show? Takeshi has suffered from much character derailment during this past decade and was due for departure ages ago. He's no longer interesting nor relevant.
Not always,at least not in Misty case.To this day people demand for her to return as main in show again.Considering how she never got proper closure never being used toi fill potential as character havbing unexplored things in story i dont blame them.Other parts of the Pokemon fandom are used to new protagonists being introduced and old ones leaving every generation. There's no reason why you guys shouldn't get used to it too.
Yeah keep dreaming,.Mays popularity had nothing to do with writers not bringing Misty back.Just like Dawns popularity had nothing to do with May not coming back.Judging by interview with ex pokemon director mr.Hidaka and Takeshi Shudo blogs writers simply rather want to use new characters instead of reusing older ones to develop them more no matter what benefits this could bring.Its formula of this show which changed not giving much chance for somene from past to become major character again.Cilan is already a very popular character, which means Brock has no chance. Its very similar to how when May replaced Misty, and May's popularity is what led to the writers abandoning Misty.