Pokemon will continue to churn out what amounts to exactly the same game with each core title as it has since the very beginning, only with gradually scummier business practices designed to carve the games up and milk as much money from consumers as possible. DLC is simply the first step towards the likes of monthly subscription fees (which we already have separately with Pokemon Home, and compare that to Pokemon Bank for one hell of an eye-opener on how badly they can exploit this), lootboxes, and microtransactions (which I suspect will come next), and it'd be passing stange - and foolish from a business standpoint - for Game Freak and TPC to not take advantage of people's willingness to shell out for this.
They won't stop at DLC, because nobody stops at DLC - like Nintendo, they're just a decade or so behind the rest of the game industry. DLC has become normalised in the gaming industry now, and whilst microtransactions are frowned upon, they don't generate as much heat as lootboxes do these days...well, unless you're Koei Tecmo, which will put $2500 worth of DLC into a single game and charge you each time you want to change a character's hair colour. I doubt Pokemon will sink to those kinds of levels, but paid DLC Pokemon are almost certainly going to become a thing within the next few generations. With people's appetite for character customisation, I fully expect cosmetic DLC to happen at some stage as well. As with everything else, you're going to be paying separately for features you used to get for free - don't tell me I'm being negative, because this is a reality happening now with things like the GTS - and, because it's comparable to everything else, they're going to get away with it. Because it's taken them this long to hop on the bandwagon, there is going to be no reasonable ground for detractors to stand on. Pokemon has DLC? So what! DLC has been around for over a decade now! DLC is the future of gaming! Get with the times!
...needless to say, pointing out that in other areas DLC adds actual value to existing games, and is not just a portion of that game carved out, will avail you nothing. Never mind that Sword and Shield were sold as incomplete products. Besides, other developers do that as well. Does anyone else remember when "season pass" used to be a dirty phrase?
Pokemon isn't going anywhere and it's not "doomed" from a business perspective, but the franchise is going to evolve to take advantage of current gaming trends, even as it remains exactly the same fundamentally to take full advantage of nostalgia and Game Freak's general lack of ability to make anything successful other than Pokemon games, and it's going to become far more anti-consumer than it already is. Game Freak want all the money and are unwilling to provide any of the value to justify it. If you think spending $60 on the game, $30 on the DLC, then paying whatever it is for Pokemon Home is bad, you ain't see nothing yet. There is so much potential for abuse in this, and if the DLC is well received - and it will be - then it WILL be abused. Perhaps we'll get a series of e-cards again, or some more amiibo which gate off exclusive features. Who knows. Whatever it is, it's going to generate a ridiculous amount of money, be immune to criticism, and be something that you would have gotten for free prior to this. And most people will be okay with that, because it's Pokemon, and Pokemon can do no wrong.
tl;dr the future of Pokemon is that you can expect to pay more for less of the exact same content that they've been putting out since the very first generation.