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Here's my thoughts on how it went down.
GF makes X&Y in 2013.
Releases ORAS in 2014.
Releases Pokemon Z in 2015 and then takes the two year period to release Gen 7 in 2017.
But TPC and Nintendo demanded a new main series title for the Anniversary.
Making them rush Gen 7 out faster, and canceling Pokemon Z.
GF also didn't expect the Nintendo Switch so soon. But due to the failing of the Wii U, they had to make Gen 8 sooner than expected as well.
USUM is just a placeholder to hold people over till Generation 8 in 2019-2020 to prolong Generation 7 from being SUPER SHORT