Late reply, but i would like to give my two cents on this:
Changing traveling companions is one thing, which sometimes involve risk too( referring to backlash after Brock departure in OI or Misty's after Johto), but trying to get rid of anime main mascot after 15 years is pretty much something which is just asking for trouble.
Getting rid of Ash and doing complete reboot would certainly cost more money than current formula where one to two traveling companions is changed. For start writers would have to change set up and structure of anime story when overhauling show and getting rid of everything for which current pokemon is known worldwide. Meaning investing in new brand for pokemon, creating new footage and frame cells for new characters instead of using currently existing ones with Ash as well coming up with completely new skeleton on which new anime would be based.
There is also new VA to hire, new design to come up with, new plots, backstory, pokemon team and route through which new protagonist would be taken being required to create. You could say how they already do that with companions but it really isn't same.
By getting rid of Ash writers series lose icon and representative with who people associate pokemon . They get rid of already successful protagonist which proved how it works attracting new generation of kids toward anime and bringing loads of cash. They get rid of pokemon mascot which is most recognizable pokemon character in world being driving force and someone who stands out for pokemon among viewers. Whoever you meet and mention this show, they immediately connect it with Ash and Pikachu. That shows how integrated within pokemon his character really is. Lastly they lose main mascot Pikachu which is connected with Ash being viewed as inseparable set.
Besides getting rid of one character costs less money than getting rid of all characters having less work on your hands.
With current formula they already have defined and established story just picking up from where they stopped with Ash going forward. ,making their job a whole lot easier. They already have anime with established brand working under formula which doesn't involve risk being widely accepted and successful among watchers.
But with new character, series would require heavy retooling coming up with whole new cast,setting and storyline. Essentially starting out from scratch, with writers being blind finding themselves in blurry waters which doesn't guarantee success.
There always exist risk of new character and show not being accepted well among viewers starting to lose interest . There exist danger of Ash departure bringing backlash turning away audience with popularity and impact he left on pokemon becoming huge after being part of it for 15 years. Do people even get idea how much Ash became integrated within pokemon and iconic after so long?
No one wants to invest in something new which involves risk of failing, if current formula already brings profit not backfiring on you. That's bad business move and recipe for disaster.
Besides just because someone journey goes for long time, doesn't mean how pokemon is anime without ending. Finale wont escape anywhere and if we watched Ash for 15 years, it doesn't matter if writers close him down now or after couple of more years.
Why not give chance to writers, to fix mistakes done with Ash giving him proper growth with character being taken forward. Why not have character go on bigger and better things after winning league growing further as person and trainer with his pokemon. Why not change some things instead of limiting ourselves, and have character age allowing to be developed more significantly, which current age limit doesn't allow? Like writers did with Ben 10 or Naruto.
There are plenty other much better alternatives instead of getting rid of everything for which pokemon used to stand for , destroying continuity, Ash and all past characters people grew to like being erased forever just to get basically rehash of already existing pokemon show following Ash 2.0 going from start all over again.
What should pokemon do instead is actually bringing more continuity starting to focus on older characters for a change(like Misty, May etc)which has potential to be done more and appeal bringing change and fresh air .Similar how anime like Inazuma Eleven go or Bakugan does creating big success in their respective fields.
Goal is to increase ratings and pokemon popularity by attracting different demographic toward anime/older fans which above suggestion would bring, while keeping target audience. Something which would bring success and money. As opposite to making new kids confused and turn away from new show after Ash(someone who they relate with pokemon) is gone ,older fans raged and frustrated after character they either grew up with or liked was ditched and replaced with inferior 2.0 version of what we already have.