Cynthia in Undella Town, BW1
"Once every few years, the Champions of each region gather and compete to see who is the strongest! An interesting idea, don't you agree?"

BW2 PWT happens

SM Kukui
"I've always valued our old traditions here in Alola, but it's time to make a Champion the whole world will recognize!"
"To think that, someday, the kiddos in Alola will be able to go from being the island challenge champion to the world Champion! And then, when we have our own Champion, they can show the rest of the world what's so special about Alolan Pokémon and their Trainers, yeah!"

Kukui clearly wants the whole world to recognize Alola so wouldnt surprise me if in USUM the league is there but Kukui builds PWT through the course of main game instead. Thats why he mentions "world champion"

Also Dexio in Battle Tree
"Terrific! I hope you'll put victory and defeat on the line against the Kalos Champion!"

I guess you could say he wants us to visit Kalos and battle Diantha someday but why wouldnt he say so then? It sounds like jhe's expecting us to battle Kalos champion there and then in Alola

PWT in USUM?