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So, a little theory on why USM will introduce new Pokemon.
The suspiciously low amount of mythicals this gen will all be released months before USM's release, which is unprecedented. There's no way that they won't have any mythicals to shill in 2018 and 2019 before gen 8. USM was planned to have new Pokemon way in advance, which is why they didn't need to put future mythicals hidden in the code of SM like every other gen, as its compatibility with USM's pokedex will be broken regardless.
However, it's also possible that they just have heaps of new forms lined up for movies and whatnot in place of mythicals. But it would be a weird move since new Pokemon are a lot more exciting for events.
