Untrue. XY > GSC for sure.
XY playerbase = maybe 75% of the currently active Ubers community, myself included among many others.
GSC playerbase = a few people popping their heads in just for this one tournament that happened to be Ubers.
i, too, can pull out arbitrary statistics! for example, 90% of your posts in this thread have been a faux, watered down version of whitequeen.
xy, even putting aside whatever word you appropriate to their playerbase, is exactly like the BW1 argument you were so against. your "75% of oras players play xy" can easily be wiggled to "75% of bw2 players play bw1", but you choose to be selective and bark up one tree. they are both irrelevant metas, and they both objectively shouldn't be included in an "official tournament".
however, I do want 8 slots > 7 slots. so i believe including both gsc and xy is a necessary evil. just throwing out the possibility of removing the more-so counter intuitive one, and possibly upping the quality of the player pool getting into UPL at large.