I agree it won't happen.
But I've been thinking about this. If Go catches a lot of legendaries, it would be super weird to have Ash win the PWC (defeat Leon) while Go sits on the side with his Zapdos, Suicune and bunch of other legendaries that could potentially defeat the entire PWC, including Ash too.
Like, how does that work? They would have to show a battle after Ash defeats Leon, between him and Go. Where Go uses the strongest legendary he caught (maybe Mew?) against either Ash's Pikachu or another ace. And have Ash win that, to clear any doubt that trainer skills matter most, not pokemon's pure power.
I mean Go's team with Golurk, Flygon and Aerodactyl is by itself already stronger than most previous companion's teams, and he hasn't even started to catch legendaries yet. It'll be weird unless they manage to make the difference between a trainer focused on battles and one focused on catching clear.