There is too much work to justify Ash battling Champions as part of an actual quest. Bringing back some old characters is not a problem at all.There is too much work to bring back old gym leaders and/or Misty when the writers can, and have, taken the easy way out with the Junior tournament.
And this is relevant how? Misty, May and Dawn all returned without Ash mentioning them beforehand. Didn't you say that Brock's mention made him less likely to return? Which was an absurd argument, but at least be consistent.Also unlike Brock who got a mention because Dawn traveled with him, neither Dawn nor Iris and Cilan have met Misty so nobody has any reason to bring her up. We know Ash never mentions old companions either.