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Having a singular version of Pokemon released wouldn't do anything to trading.
People nowadays trade for a wide range of things, like Event 'Mons, Items, Egg Moves, Competitive Bred 'Mons, etc. There's also things like Starter Pokemon which where you have to pick one and forgo the others. Or Pokemon who evolve by trading. Having a handful of version exclusive Pokemon to trade is pretty trivial reason to sell the same game in a pair, at least in the modern games.
