The Pokemon anime aired in Japan back in 1997, taking over a year before it made its way to the United States on September 7, 1998. This notably points out that Pokemon technically just turned 21-years-old, meaning that many who watched it in their youth are now in their mid-20s to early-30s. But love for this franchise is not something that goes away easily, as clearly seen by those who are clamoring for the upcoming release of Pokemon Sword and Shield. Yet, it doesn't seem likely that those who still play the games watch the anime.
While the Pokemon anime did kick off the long-running trend that transferred to trading card games, video games, and mobile titles like Pokemon GO and Pokemon Masters, it's more designed for children. In other words, many fans are outgrowing, if they haven't already, the kids' show. This may be one motivator for the upcoming reboot of Pokemon that may not have Ash Ketchum from Pallet Town in it.
Before moving forward, it's worth highlighting that this is mostly speculation and conjecture. No one has cast the final die as to whether Ash would appear in the show or not, but since he didn't appear in the trailer or in any marketing thus far, the rumors are not without a base for contention. Assuming that this does prove to be true and that Ash is out of the show, there's likely to be a sizeable backlash, but that may not be necessary.
A Pokemon anime without Ash Ketchum doesn't seem right, as the perpetual 10-year-old has been on his journey to becoming a Pokemon Master for over 20 seasons now, but once again, those likely to complain may not even watch the show anymore. After all, the overarching plot is the same. It's clearly not about the destination but the journey, as Ash Ketchum hasn't been able to become a Master in over 20 years. The idea that he is still 10 after everything he has been through is absurd, and while the suspension of disbelief should definitely play a role here, it also begs the question as to whether it's time to replace the hat.
Replacing Ash Ketchum would have a long-lasting influence on the current and future Pokemon fans. It's unknown territory and dangerous. It's possible that this new protagonist wouldn't click with audiences, and whatever partner Pokemon they choose (it shouldn't be Pikachu, even though its face is more iconic in the franchise) may not have a big enough influence. But done right, it's possible that new fans come and go for another 20 years for the franchise.
While trading off Ash Ketchum doesn't sound right, Pokemon lasting for 40+ years does. It's not even that much of a hyperbole, as Pokemon tops the 25 highest-grossing media franchises of all time. Whether the franchise can keep the momentum for that long remains to be seen, but it's hard to imagine 50 and 60-year-olds sitting around watching Pokemon, at least specifically for Ash Ketchum.
A new trainer with a new theme could breathe new life into the franchise, which would also fit with what little is known about the reboot. As it stands, it seems that it will focus on the Galar region (if not other areas as well) and include Pokemon from all generations. Recent art for the anime showed Grookey, Scorbunny, and Sobble, for example. While many of these changes do make sense, there are two questions that would be hard fixes.
First, what would become of Pikachu? As aforementioned, it wouldn't seem right to pawn Pikachu off on a new character, even if it was a different Pikachu. The yellow mouse Pokemon has been iconic with the franchise since its inception, so much so that many later Pokemon were based on its design, and it's hard to imagine a successful iteration of the franchise without it. Furthermore, even if was even attempted, would the new Sword and Shield starters - or any Pokemon for that matter - be enough to match Pikachu's legacy?
It doesn't seem likely that Pokemon would be quickly get rid of Pikachu, but the current rumors put the Electric-type in an awkward position, which builds up to the second unanswered question. Would replacing Ash as the protagonist mean that he's completely out of the anime? He could take a secondary or background role that would keep him and Pikachu in the anime, even if it was far and far between.
Ultimately, the rumors are just rumors for now, so it's uncertain which direction it will take. Fans should wait for confirmation as to the protagonist of the Pokemon anime, and be willing to give the new protagonist a chance if the rumors do prove true.