That's a huge contradiction. Besides Misty's doing just what she wanted to do back at home; be a competent and respectable Gym Leader. Misty too suffers from virtually the same problems as Brock that she can't easily be given a goal like badge or ribbon collecting. As a Gym Leader it'd seem a moot point to have her earn Gym Badges and she isn't girl enough to be a Coordinator.
Are we even watching the same show?
Misty left the gym in order to become worlds best water trainer/water pokemon master and didnt want to leave the group in the first place becoming gym leader out of obligation when her sisters left her without choice("Gotta Catch Ya Later" episode).Her goal was never to become gym leadere but water pokemon master.
Nevertheless after that judging by some episodes after her departure(like "A Date With Delcatty"original version not dubbed one,and to some extent "A Real Cleffa Hanger") she showed displeasure with her current position implying/showing how she would rather want to travell,and isnt leaving gym because of her incompetent sisters to who she doesnt thrust enough.
Also being gym leader isnt enough to achieve what Misty wants to become and the only way to become stronger is by travelling,exploring new ideas,competing in competitions for water types similar to Whirl Cup,and battling against stronger trainers.So this is perfectly valid reason for her return.
Further to that in case of her return you cant judge how much she would receive screentime.Times are changed and writers have improved so hipothetically if this happens they would give her something to do,like competing in water based competitions being one option out of all possibillities).You know collecting badges and ribbons isnt the only thing which exists in anime.
But yes i made one mistake not being enough precise,what i meant is that i find Misty being more interesting than Brock is.Like i said this is what should happen in my opinion if Brock ever gets replaced(which probably wont happen for already mentioned reasons).