I don't mind if you turn it around, because we do not fault XCM for being a different genre. So why fault Advent for changing its protagonists?
Yes, but XCM, despite not being a numbered sequel, still retains the same protagonists. One could arguably say that it does a better job of highlighting the title character than the main series.
My point is that you were implying a rather rigid standard as to naming trends where none exists.
ZXA had no choice but to devote the entire game to the new protagonists for the sake of both plotline and gameplay.
Programming minor stage tweaks based on character selection is a fairly simple and well-established task in gaming. And quite frankly I think a "what-if" scenario of MegaMan Model ZX challenging Albert would be a highly valuable one; more so than Vile in MHX, and certainly more so than the Navigators in X8.
See, Advent doesn't just shaft Vent/Aile in gameplay, it de-values them by MegaMan Model A defeating/A-Transing them and implying MegaMan Model A as being the sole worthy challenger to Albert. In no way am I against the focus on Grey/Ashe, they add plenty to the story. I simply feel that failure to provide Vent/Aile as even an unlockable option was an inexcusable mistake.
That no sequel as of yet exists is no fault of ZXA.
"Fault" is irrelevant. I am not assigning blame, I am citing your use of situations that may or may not exist as the sole basis of your views, which is simply flawed logic. You cannot know that the next ZX game will or will not be titled, much less treated as, ZX3, or that the next Vent/Aile game will or will not be a numbered sequel, until it actually happens.
Just because the second game in a series is not numbered in its title, in no way whatsoever does that mean the third game will not be numbered 3 in its title (see Toejam & Earl 3).
It is not that X does not have more of a story to tell, it is simply that they chose no to tell it. Both Irregular Hunter X and Rockman Zero confirm this. Shifting the focus to Axl and Zero whilst shafting X was their conscious choice.
Interesting choices. MHX is a remake of the original, Zero is a new series with a century or two of backstory to cover.
See the connection? They're both beginnings. Little is known of X, lots of out-of-game setup has been going on, so yes, there is PLENTY to tell. But if we're at the 4th or 7th entry of a series respectively, the mere fact that the pre-existing protagonist is well established says that naturally there is more to say about the new one. By no means does it have to stay that way as the series continues; I for one think X got far more significant character development in XCM than Axl or Zero. Same for X6.
Not that that is necessarily bad, I don't even fault X7 for it, considering that it was needed for the plot. But marketing as a direct sequel to X6 was just bad; they really should have been more open and clear about these kind of deviations.
From the very first trailer they were clear that the game was 3D, in and of itself a severe deviation. As for X, I really consider that (in gameplay) to be inconsequential. He is unavoidable (which puts him in a better boat than Zero in X6), and his power more than justifies his delayed entry. It's rare to see X as the high-attack character when compared to Zero. My only concern as a matter of focus in X7 is why X got so little cutscene time, but quite frankly X's general character in the game is far more troubling than any cutscene absence could ever be.
Even numbered sequels can heavily deviate from their predecessors (Zelda 2, Metroid 4). It's considered risky, yes, but it's by no means unheard of.