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People are setting themselves up for disappointment. These past/future forms will not be separate dex entries, but function like megas/gmaxes. We'll get maybe 2 dozen and then they'll disappear in gen 10. Put together that another 20-30 are going to be the regional fakes, we're looking at far less than 140.
Khu said to expect XY dex + megas, so we're going to be closer to 80-90 new + past/future forms. I'll happily be wrong but to expect 130-140 new mons when they've purposefully been doing smaller gens since 6 is setting yourself up for disappointment.