So can we accept that X is Charizard's true mega form and Y was just tacked on and irrelevant now? Do Nintendo even pretend Y even exists anymore? I'm not saying I dislike Y, in fact I think it's the better form but it's pretty neglected

If promotional media requires the use of a mega charizard form I've only ever seen X be used

In the special Red's charizard mega evolves into X

In the anime Alain, Ash's big rival in XYZ, has his whole identity built upon his signature Charizard X

This is debateable but X, the title which gets Charizardite X, is the title which Nintendo/Gamefreak has a slight bias towards being the default (Like Red, Gold, Black etc) if we use other media like the manga and the aforementioned special as reference