While I would prefer to keep most of the currently existing depth in Pokemon myself, the franchise is 24 years old, about 3 months away from 25 years. You are talking about it like the fad phase of the franchise hadn't died by 2002. You are confusing casual fans for fad-chasers. On top of new blood in the form of kids, there are plenty of people that have bought every game since Gen I that could give less of a hoot about the complaints thrown in Sword and Shield's direction. I know plenty of those in real life. I understand if this being primarily a competitive/non-casual website makes this seem not likely due to the people we regularly see on here, but it is the reality of things.Focusing exclusively or heavily on the casual audience also does not work that well in the long term in terms of sales. Yes, it would increase sales even more-in the short term. But that's the thing: casual audiences will not stick around for the long term. For many reasons stated above, making the games to focus on casuals means that you focus on an audience that will gain interest quickly for the short term, but then lose interest just as quickly. At the same time, the content would not be enough for more core players (and by extension, hardcore players), and it would eventually heavily alienate them. This isn't going to work well long term because in the long term you will lose the core players quickly, and you will eventually lose the casual players at some point because Pokemon won't be "cool" to them anymore at some point, and then it will struggle to gain the interest of either crowd. Ultimately leading to them actually getting less long term sales as they will eventually fall down.
Look at the Wii U for instance. One of the big reasons it failed so miserably commercially is not just its bad marketing (attachment to the original Wii name), but because of the way the original Wii itself was marketed. The Wii was a prime example of a thing that was designed to heavily pander to the casual crowd. Yes, it was financially extremely successful, but it was so heavily pandered to the casual crowd that it alienated most of the core audience who did not like it, perceived it as a "kiddy console", and shunned it in lieu of the PS3 or 360. So while it sold many units, most of the people who actually bought the Wii (note, I am not saying all that there were no core/hardcore Wii players: there were still core players who enjoyed the Wii and its first party stuff, but they were a minority) ditched it in a few years, it became a dust-gathering thing in the closet of many homes, and most people moved on to the next fad like Angry Birds or whatever was popular later. Then the Wii U came out, and people thought it was an add-on to the Wii because of the way it was marketed. Why was that a recipe for disaster? Because the Wii name had more or less died out: no one in the casual crowd cared about the original Wii anymore, and the core players had a bitter taste in their mouths because of the Wii's casual pandering alienating them from Nintendo and especially the Wii name. Ultimately leading to a struggle for the Wii U to sell well in its entire lifespan, and despite being a great console with a lot of awesome first-party exclusives, it sold poorly and died an early death.
This is relevant in that if you were to pander Pokemon to make it focus exclusively on the casual crowd, it would not only homogenize the franchise and make it unable to improve, this would lead to two things: the core players who are invested in Pokemon in the long term will get mad and start complaining, and after a while will dissociate with the franchise out of frustration, and the casual players will lose interest at some point because to them, "Pokemon" will one day cease to be "cool" to them and they will move on to the next cool fad or whatnot. Meaning this would lead to Pokemon titles eventually struggling to succeed in the long term as future games will eventually start struggling to sell well, and eventually you'd start reaching a point where it'll start having repeated failed titles over the years. Point being, it's not a good strategy for the long term and it will only garner in higher amounts of cash in the short term but failure in the long term. Maybe it will take a loooong time for casuals to lose interest, but still: it will happen at some point in time.
If Pokemon wants to survive for the long term, and actually work to be better quality in the long term, at the same time it still needs to acknowledge the criticisms from the "loud minority" in some way, shape, or form. The core and even the hardcore audience is what will keep the franchise sustainable in the long term because there will be people who are interested in sticking with the franchise for the long haul, and they will want the franchise to be better and the games to be better quality. It needs to work towards being better quality for core audiences if it's going to stay around long term and not just rake in cash for the short term, because core players can and will be fed up at some point otherwise, and casual fans will one day lose interest too because the "fad" value of Pokemon cannot last forever: the fad value will one day die and not many people will care about it anymore.