But what happens when League ends? When tournament play begins?
You're fairly accurate in your description of the troubles casual unlimited format players will have when tournament time rolls around.
But I think you must consider the idea that not all players are going to play in tournaments.
It was a hard idea for me to adapt to considering my local area. We had some premier events come to our area and almost nobody from the local area came to them. The premier events stopped coming after a while and most of the people at league were not phased.
To you and me, this game may seem to revolve around the tournaments, the modified format, etc. But there is a large, possibly larger, community of players spread across the nation that only take part in the Pokemon League. They aren't interested in tournaments. They aren't interested in becoming competitive. Heck, they may spend half of each league session playing Pokemon Rumble! (true story)
We just need to accept that TPCi may spend some of their resources, not all, and I'm not saying more, but some, on this demographic. These guys (and gals) gain a lot from great League promos and it doesn't matter to them when those cards get rotated out of the Modified format.