The writers don't care about the older audience that much. This is why the Misty backlash has fallen on mostly deaf ears. Whatever the last season is the writers are bound to have it focus on Ash and him alone and at the very least bring in two characters from Last region for regional travel companions because they can still market whatever game the last region is in. Neither Brock or Misty are marketable for anything gamewise which is what Pokemon in general thrives on.
In order to advertise new games you dont need new characters.Older characters can promote new games through new pokemon just fine as any new character would.
As we get to see with Ash,Brock for all this years,Team Rocket etc older characters can have marketing value too being possible to promote games through them along with new characters being showed through guest appearances like they did with HGSS remakes for example with several game protagonists from main games being ignored.
Nintendo and Game Freak could care less who is going to be in cast as long their main product and source of money(new pokemon)are being properly showcased.
In fact most characters who were replaced werent because of advertising purposes but because writers want to add change through new "eye candies" according to interview with ex director mr.Hidaka or because they arent well received(like Traceys example).
Even if Brock and Misty do come back what are they going to do, be background again? Force a new goal on them to make them relevant to the last region? Lets say they decided to expand on Misty's water goal or Brock's goal as a doctor, again why is this more important then May/Dawn's goals who many wanted to see win a GF or Max becoming a trainer? Or Iris with her goals and Cilan with his? You cannot please a entire fanbase.
If Misty and Brock comes back to accompany Ash on his last journey writers would probably give them some purpose to travell on his side.
Ash is main protagonist and he will always receive most of focus(even co-star like Dawn falled behind abit) but if Misty and Brock would return they would most certainly receive some focus on themselves too through which writers would concentrate to tie up loose ends in their stories giving them character development.
Their focus would be along the lines of what Dento an Iris are receiving with Misty probably getting more than Brock due to fact of being trainer meaning she would battle more.
Speaking of pleasing entire fanbase by time last season comes Iris and Dento are likely going to depart long time ago.
You cant satisfy everyone but majority of fandom would be either pleased or neutral welcoming if original trio gets reunited for another ride.
By neutral im referring to target audience(small kids)too who are more open minded than older fans are not being so critic toward show.
About statement of "why focus on Misty and Brock should get priority",point in here is not in who is more important but in what way would be best to wrap up long lasting show like pokemon is.Like i said before it would make more sense to close down series with first Ash traveling companions forming group with who everything started adding for bigger impact.
Thats not to say how Max,May etc wouldnt get some focus too,because they would with writers giving them chance to shine prior to end,its just they wouldnt be in last season as main characters with there being space for only one group which would accompany Ash on final adventure in case writers decide to prepare end of this show in that way.And in my opinion that would make more sense to do with first trio.
Reuniting the old cast would just make the old fans of the original saga happy but what about the AG, DP, and Best Wishes fans who like their characters more?
This may seem strange but many people who watched AG,DP or best wishes wouldnt mind if original trio gets reunited from what i noticed.Sure they would rather like their favorite groups to see light of the day but if first group writers revive most of them would be either neutral or would welcome it.
Very few would be bursting out of anger with pokemon fanbase usually being open minded once writers something introduce in show.
p.s.About suggestion of Ash traveling for last saga alone there is a reason why writers add traveling companions.Having Ash going for 60 to 80 episodes alone would be boring to watch with no real character interaction existing or support.
It makes more sense if he gets accompanied by friends on his last journey on screen.
