i'm gonna have to disagree with this. i don't think they would just ban something because of it's status. they still allow some legendaries that are slightly more powerful so why ban the main legends if they aren't any more powerful?
They have always banned selected legends if not all of them. The Event Legends and main legends are banned due to how you get them while all other legends are always optional. The lesser legends even the ones with 600 base don't have the same movepools as others and can be seen as weaker compared.
What ever reason Gamefreak has, if nerfed all legends stand to get banned like they were back in Gen 2. Most of the tournaments didn't allow legends in Gen 3, but when Gen 4 introduced its large lot a lot of the lesser ones started being allowed. If Gamefreak evens out the legends they'll just ban all of them again.