I don't think you wanted to use Yu-Gi-Oh! as the example there. For one, since 5D's YGO has been set in different universes/time periods from the DM-GX continuity. There haven't been cameos from previous main characters in nearly a decade, with the last team up happening in a pseudo-canon movie, Bonds Beyond Time. Side characters appeared in Arc-V as fanservice, but they ended up hurting the story overall due to incompetent writing.
Second, YGO has been the posterchild for bland, irritating main characters ever since Atem's story ended. That's why all the movies, and most of the games, have been focused squarely on Yugi and/or Kaiba.
Like it or not, Ash is the co-star of the Pokemon anime alongside Pikachu. Investment with him and his struggles/adventures is why people actually care about the Pokemon League, which in the games is a joke and wasn't a major story focus until Alola.
I remember when Pokemon Origins came out and everyone was thrilled with Red as the protagonist for the first episode. But by the second episode, people realized that he's Ash without any of the charm, struggle or relatability. I guess you could call the stupid game-like qualities of that universe "struggle" but it wasn't in service of entertaining or emotionally engaging the audience.
Bottom line...Ash is an institution, and while that's never a good reason to keep a character tied to the cast, the anime could easily do more with him to keep new fans interested. Ash hasn't changed in 20 years but neither has Goku, and the main difference between them is one character always loses, while the other always wins.
Of course I'm not talking to do it like Yu Gi Oh in that sense. About Yu Gi Oh you're completely right, but in Pokemon it is different. Characters could leave a lasting impression if each one implemented a different kind of growth. Be it a shy trainer into a confident one, a brash trainer learning with time to think things through. The possibilities are too many.
About the YGO comparison, just talking about doing new characters that can develop and interact with the Pokemon world without needing to change an already existing one, causing discontent among fans. No need to mess with timelines and worlds like that. Pokemon already assumes from the games that everything is connected.
Ash is a pillar of the Pokemon Franchise, but they could do it like in adventures that the character pop ups from time to time with new Pokemon or evolutions.
About the risk of characters being bland, that depends on the work the writers put. I work in this particular department and I'm telling you, creating characters is annoying as hell because you run the risk of making them too bland, too gary/mary stu or simply secondary to other character. It just depends on the one they place in charge of creating them. YGO has the problem of making them too extravagant to fit the 'character type' they want. And that sells for the franchise I guess.
Pokemon Origins had the problem of being a series of OVA that simply did not let the character develop at an even pace, but it actually follows the basis for every generic shounen character. Starting weak, not knowing what to do, learning and eventually triumphing over everyone. The problem is that it becomes too evident with the runtime being so little. That it was used to promote GEN VI is the main problem, but i'm getting out of track here.
Comparing Ash to Goku is your worst bet actually. There is no more hated character than Goku for being so predictable. That they keep squeezing the series with repetitive stories proves that Dragon Ball needs a change. In DBZ the point where things could have gone a different route would have been to keep Goku dead and Gohan as a main character. Toriyama caved in when people wanted Goku and simply condemned the series. In that sense, Pokemon became too repetitive and the writers tried to make a big change with the animation and the tone of the series. As they changed Ash for that, people revolted. It's actually very simple to see that, even if in Japan people like the series way more than the previous ones, but i'm talking for occidental audiences.
Pokemon needs a change in their paradigms because people will lose interest.
