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Why doesn't Game Freak consistently improve on the Pokemon formula and keep moving forward with new ideas while keeping the old ones that worked?
Every single gen removes something substantial, either permanently, or temporarily to bring it back as a "new" feature in a later generation game. Gen 3, for example, cut the entire day/night and weekly event cycle. Emerald to DP cuts the Battle Frontier, only to bring it back for the third version as something "new", and Gen V removes the online co-op factor of the Battle Tower (or Frontier), with the feature never returning in any game since.
Third versions all have content that should have been in the game from the get-go. If GF didn't bother to rush game releases, GS could've gotten a female protag option, RS would've had the Frontier, better double enemy teams, rematches, Steven's final battle, and the space station battle, and the same with DP and SM and their third versions, rather than being packed as DLC for the price of a full game.
On the topic of remakes, every single Pokemon game holds up, even Gen I with all its bugs. The Kanto and Johto remakes could be partially excused for mon collecting and compatibility reasons, but even then Johto's were somewhat redundant when FRLG, Emerald, and DP all have a substantial amount of Gen II Pokemon. The entire Johto/Hoenn experience could easily be relived just by playing the original titles, and the same goes for any other potential remakes. With Virtual Console being a thing, any of the old titles could be released anyway, so people who don't have the games could experience it, and GF could use this time to refine their games even more rather than rehashing the same game. Any of the remake-exclusive content could be added to the next generation; BW could've had the Pokeathlon, and SM could use the Dexnav.
Of course, I know the real reason as to why this doesn't happen is because GF's current method makes money.