The general rule when it comes to live service games is “you (almost always) have to support everything you’ve ever made forever”. You don’t necessarily have to make more of that content, but whatever already exists needs to keep working. If there’s a game mode that you added five expansions ago, it still needs to work. If there’s a feature that’s been there from launch, it still needs to work. And if some number of players paid money for it at some point, it definitely needs to work. This is why long-running MMOGs often have weird redundant gameplay systems like SWTOR’s old spaceship rail shooter with its gear and upgrade path, and its other spaceship shooter game mode (Galactic Starfighter) with its other ships, gear, and upgrade paths.
To my understanding, the main reason the launch companions have different post-timeskip versions is because the new post-timeskip versions don’t have to support all of the old companion-specific quests and old gift/affection system baggage that were built for launch, so it was much easier to allow them as companions for more player classes.
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