Interesting question, but when we take a look at bigger picture it makes perfect sense why one of older traveling companions was allowed to accompany Ash in future sagas after Johto ended.
In many long running shows unlike its practice with pokemon , cast which started anime in first place sticks with viewers and storyline until very end. We can witness to that with One Piece, Naruto, Winx, Fairy Tail, Inuyasha etc, etc.
With purpose of delivering balance into timeline and anime following. Established main cast which creates brand about themselves and stands for animated series through which storyline, development and adventure is told. Forming emotional connection between viewers and main characters promting them to stay tuned into anime and have reason to continue following their journey.
Thus bringing identity to show that way.
Brock stay for Hoenn and Sinnoh was to provide stable additional character whose formed friendship with Ash, history, past experiences and influence left on Ash would transfer itself on next generation.
I don't know if others noticed, but when Brock was around there were no resets of Ash skills and character progression(unlike Unova did). Making it seem like pokemon show still have soloid story which builds on itself gradually.
Giving someone else beside Ash with who audience could associate pokemon series, keep continuity stronger due to strong bond, story and all kind of good or bad moments he went through with Ash.
Attesting to previous development and newly acquired skills Ash developed over time explaining to new companions and new viewers some things thery werent aware of which took place in previous seasons. As such serving as insurance that collected knowledge and development created before wont go down the drain.
Hence why in Hoenn and Sinnoh pokemon series had stronger continuity, connections to past with bridges not being burned and more coverage on previous important characters and journey.
Something which is hardly true for BW, but even XY as well as far as attention to history and past important events really go.
Not to mention longer stay allowed that Brock character flourish to more satisfying degree.
Learning new things about his past, family and relationship with parents or younger brother Forest. Found out how he got his first pokemon partner(Onix). Caught all kind of new, interesting pokemon, went through some endearing development. Gathering new info and experience in raising up pokemon putting to test his attainment as pokemon breeder. As baby pokemon like Bonsly, receiving his own egg(Happiny), taking care of injured pokemon like Sharpedo, Bagon, Nuzleaf, Dawn Pachirisu etc showed. Along with helping to calm down conflict and urgent situations regarding enraged pokemon with his wisdom, patience and adamant nature.
Sprinkled with lot of funny moments, comedy and embarassing situations Brock awkward behavior and moments of irrationality brought in group dynamic.
Brock stay also allowed to decrease impact Misty departure caused among viewers, making it easier for fans who kept watching show to get accustomed to cast replacing anime staff opted to go with for new generations.
With removal of one extremely popular character at that time berng already big enough shock, to risk it any further and remove another heavily appreciated traveling companion and cause further decrease in popularity than it already happened.
BUT even than, when Hoenn started i always felt pokemon anime would be much better and more rewarding for storyline if Misty was kept instead of Brock to strengthen continuoity and bring stability to main cast through her presence.
She is more likable as character having bigger depth to her emotions and personality imo, has funnier pokemon, more sad backstory(left by parents at very young age and neglected by her sisters and everyone else which left emotional scars on her wanting to get noticed and get love, support she never had in childhood). More interesting career centered around battling, catching new pokemon, training and entering tournaments to reach E4 potential and become water master.
Just Hoenn alone was full of
opportunities and water themed plots.
Something which would imo bring more excitement and unpredictability to Hoenn and Sinnoh series than Brock really could due to limitations his plots, breeder goal etc had attributed to themselves and weaker predispositions through which he could grow to more meaningful way in personality and emotional sense.
Because Brock is better than Misty.
Also, when Brock first left before the Orange Islands, fan backlash prompted his return. When Misty got the axe, hardly anyone except hardcore shippers and fangirls cared after the first couple months. Truly, Brock should still be there since none of his replacements have fared better than he did.
In that case we would have to go with presumption how only pokeshippers and "fanboys" liked Misty character, which is laughable and not true at all.
Im not sure how you expect that i take this statement seriously? It was obvious how backlash and controversy Misty departure left on pokemon fandom was 10 times worse than Brock short hiatus for Orange islands achieved.
With number of fans who were disappointed, saddened and shocked that Misty was replaced vastly outnumbering those in former atempt pf replacing character case. Followed wit huge amount of complaints, petitions, campaigns and all kind of actions taken by literally thousands of fans in attempt tobring her back. They failed, but this revealed huge dedication and support Misty character enjoyed from fans.
Reason why Brock returned after Orange islands wasn't because fans wanted him back.
But because:
-writers realized how everyone were fine with Brock look with there not existing any racial issues they feared he might caused.
-Tracey wasn't liked enough to warrant his stay
That kind of utility was needed during Hoenn and Sinnoh because we had two co-stars with tangible goals already competing for the viewers attention. Anyone else would feel like a Third Wheel.
I believe that controversy surrounded around pokemon breeding and fact of Brock storyline not having much ties with training, battling or competing also played relevance in how much attention and exposition his character could had received.
Well it certainly played factor in whole thing.
Brock's role certain diminished as time went on. It seemed that the less the writers used Brock the more they realized that they didn't have to keep him around, and they could just shift his utility onto other characters.
I would agree, if it weren't for fact that Brock got more episodes dedicated to himself, more development about his breeding career through exchanging experience with other people of same profession and nurturing hurt pokemon, more battle screen time and focus through which you could feel his presence in Hoenn/Battle Frontier building on friendship with new companions(especially Max).
Than it was case with Johto.
Sinnoh is another story though.
