To clear the air, this is not a comparison of the Digimon and Pokémon series. They're both incredible series that have continued to thrive despite changing times and the occasional new competitor. While Digimon has never had the same following that Pokémon has now earned worldwide, there is still a dedicated fanbase. The striking part of this fanbase is the number of adults that are a part of it.
This is a direct result of the way the franchise has continued to grow with its audience. Whereas Ash has been 10 for a few decades now, Tai and the gang have aged into adults. The most recent release for this cast of characters, Digimon Adventure: Last Evolution Kizuna, even goes so far as to warn of the final adventure for Tai and Agumon. That'd be like The Pokémon Company announcing the next movie centering around the death of Pikachu and Ash trying to find a new starter Pokémon. This isn't the first time that Digimon has seen the Digimon say goodbye in some form, but this is the first time in the series it has been done so with solemn finality.
There are a few key differences in the approach between these two franchises. Looking more closely reveals some interesting patterns that have both yielded success, but in very different forms.
Ash: The Immortal Child
Ash has been 10 since 1997. Let that sink in for a moment. He's still got that same Pikachu that refuses to evolve no matter how many Pokémon Leagues he fails to conquer. At first, it doesn't make sense how Ash has been able to defy the natural laws of science. Some have theorized on how this is possible though, like the theory that involves Ash's exposure to Ho-Oh in the original episode of Pokémon.
The theory posits that the Pokédex entry to this legendary bird holds the key, stating those who catch a glimpse, "are promised an eternal happiness." It's a nice way to look at the franchise and mostly works, however it also falls flat when you consider that nobody else in the series has aged either. Has Ash's Mom, or Brock, or Professor Oak also seen Ho-Oh? That's doubtful.
In all honesty, the real reason (less magical though it may be) is most likely that the anime really has always served as an entry point to the merchandise of Pokémon. Keeping Ash the same but updating his look for younger audiences keeps him fresh and approachable to new generations without ever making the series unrecognizable to older fans. 90s children are mostly adults now and starting to have children of their own. Seeing Ash with his Pikachu more likely brings up thoughts of "Ah, I remember loving Pokémon! I'll share this with my kids." It's the equivalent of a parent saying "Music was different in my day!" except that they can't say that because it may look a little different and there may be new faces, but it is essentially the same base experience.
The support for this theory is in the amount of copies Pokémon has consistently sold, constantly hitting more than 10 million units in sales. That's just on handheld systems too. It remains to be seen how far the most recent entry, Sword and Shield, will go after an already seismic release. As older fans grow out of their Pokémania, new fans discover the show and take their place.
Without sounding too cynical or acting like Pokémon is just after nostalgia-fueled dollars, it's important to remember at the end of the day that this is the part of the franchise aimed squarely at children. Adults can get their competitive fix in the games and card games if they want a more intense experience. The whole point of the show is to give kids a crash course and welcome them to the modern obsession known as Pokémon.
Digimon And Evolving With The Audience
Digimon has taken a decidedly different approach to Pokémon. The first series follows a group of kids at summer camp who are transported to the Digital World. The second series introduces a new set of Digi-destined heroes, but still keeps that original cast around as mentors to the newer faces. There are a few seasons that break away from this original group of heroes, but the story has once again found its way back to Tai and friends in recent installments. It's not just that these characters come back, but also that they continue to age with the audience.
Tai and friends went to summer camp at the same age most of the kids that watched Digimon in the '90s would have been at the time. Upon their return in Digimon Adventure 2, they've all aged to high school, with the new main cast still in middle school. This also follows the audience's age while also allowing for a slightly wider access point to those who've never heard of Digimon before. Digimon Tri brings the main cast back during college and in their early 20's. Digimon Adventure: Last Evolution Kizuna sees Tai enter full-on adulthood. Each time the series comes back, it's directly dealing with the fact that its audience has grown and expands on the ideas it brought forward previously.
Digimon Adventure: Last Evolution Kizuna raises the stakes further than ever before. Sure, Tai and friends have had to say goodbye to their Digimon in the past, but the sadness is usually undercut with a glimpse of hope or a hint that it's not really the end for the friends. Last Evolution Kizuna does not pull any punches. Without giving away too much, the plot centers around the Digi-destined realizing their time with their Digimon has a definitive countdown on it. To make things worse, making their Digimon battle will make their remaining time even shorter.
It's asking the audience a poignant question: when faced with a higher challenge or responsibility, how do people handle losing the things they loved while growing up? It's a statement on responsibility, with Digimon representing a link to childhood pastimes. Everyone eventually has to grow up in some form or another. The ending is heartbreaking, especially for longtime fans. However, there is a bittersweet implication that, just like in life, rising to meet challenges and doing what's right can lead to eventually having the freedom to revisit your formative passions.