Dexit is the very embodiment of this dilemma. The Pokémon themselves have largely been exempt from the regular culling of features. New abilities have stuck, new moves are always kept, items are carried over with extremely few exceptions, even moveset changes tend to stick between generations (bar stuff like Metal Claw for Charmander). The features related to battling have always been expanded, never reduced. Every new Pokémon and all of their attributes have always been carried over. A loss here hurts
badly, since it's unprecedented.
But the same economics apply here as with other features: they can't keep adding stuff forever. At some point, something has to change. It was inevitable, just look at the Tumblr post
p0ip0le linked to. Making one change such as migrating engines or adding new animations to all the old Pokémon eventually becomes an insurmountable task.
However, the change they went for wasn't of the fundamental sort that would finally let them transition to the "every game is a separate, self-contained adventure!" model they want to have. Instead, they seem to have kept all of the features from before, but only for half the Pokémon. Essentially copying over what they already had, continuing down the exact same track as before, but stopping short. The model is still one of accumulation, but they've chucked half their content pile out of the window before continuing to pile on new stuff. This is the sort of thing fans will notice. "Hey, you brought over 'mon A from previous games, and it's the same ol' 'mon A as we've known since forever, but why wasn't 'mon B brought back? Last time we saw them, they both looked essentially the same as 'mon A does today, so why wasn't 'mon B transited?"
I think if they really want to successfully become a franchise with self-contained installments, they would have to start with a clean slate. Chuck the
entire content pile out the window and build stuff from scratch. Making games that do the "be their own thing" thing so well that they make up for the loss of the entire cumulative content from the old design model.
This was the hope many held out for Sword and Shield. It could have been the perfect opportunity too. Ending the handheld era and going into the era of console power with console-level graphics, console-level animations for the handful of returning Pokémon, and a total overhaul of the gameplay. But instead, they kept relying on the models from the handheld days, with the same old animations, the same old gameplay, graphics that look rather underwhelming compared to those of the other flagship games on the Switch (which in two notable cases were ported over from the
previous console generation), and in general it really shows that they've iterated on the Gen VII games. But they did the "handful of returning Pokémon" thing anyway.
So yeah, a comparison with the old games is a rather inevitable part of the release of a new game, and I think Sword and Shield fall short. The stuff they add comes across as yet another gimmick, while it's the
lasting accumulation of features that fans love the series for. They continue to follow the same design model while slashing a large part of the existing content and features. No wonder they're sparking outrage. There was always some disappointment in every cut feature, disappointment they had to compensate for somehow, and now they're cutting out a major part of "the game" (that's the issue in a nutshell - to us, the Pokémon core series is all one game) without taking similarly major steps to compensate.