We will never get the Battle Frontier again.

I don't think the problem is the development time or that Masuda is allergic to post-games. I think the reason why it wasn't in ORAS and it wasn't in BDSP and why it won't be back ever again is that online battles exist and thus a BF is not needed.
Back then, (during RSE) internet wasn't a thing. Online battles didn't exist. VGC didn't exist. So they did the Hoenn BF to keep players entertained because playing Pokémon was still essentially a 1Player experience for the most part.
During Gen 4, the internet was more widespread, but not to the levels it is nowadays. And online battles weren't as popular. But in this day and age, they don't think we need (or even want) a BF because if you want to battle, you just connect online and find someone to do it. It doesn't even have to be your friend anymore, or need to add them or anything. So they don't feel the need to do anything more than a Battle Tower or its clones (Maison, Tree etc.) because it is simply not needed.

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