Doppelgänger
Superancient Member
No, the League, and Ash's purpose in the region, is always revealed in episode 1.
DP's episode 1 was about Dawn. Ash didn't sign up for the Sinnoh League until a few episodes later.
SM Ash didn't give a goal until episode 2.
These are still "early series" examples, but the point is Pokemon doesn't follow strict rules like what you seem to be implying. The league is only a loose structure for journeying around the region, and tends to be copy-pasted between regions.
The "main plot" tends to reveal around what we would recognize as the first season or series finale:
DP051 - Ash obtains Chimchar
BW023 - Team Rocket vs. Team Plasma at the Desert Resort (cancelled)
XY023 - Mega Evolution Special I
SM043 - Pokemon League announced
Consider, the Necrozma arc occurred around 1 year after the Kanto trip and Aether Paradise arc. The Pokemon League, which is named after that Necrozma event is taking place 1 year after it.
What is building up to what, here?
And the League has always been the main event instead of a side show. Even in XY, where it was part and parcel of Flare.
It isn't even the main event in the games. Nintendo considers catching the legendary mascot the climax of the story, regardless of when it actually happens. This has been true since Generation I.
The league is what people care about most, but that doesn't change that outside of anime Alola, it's not intended to be the main draw.