True.
Mind you, the counter to that then becomes "Oh people don't really care about these legacy/spin-off characters...they only care about the originals so you need to change the originals for representation". Which...I hate to say does make a kind of sense. But also leads to the unfortunate implication that the only way you can make, say, a successful black character is by 'hijacking' the popularity of a white one.
But to your point about the X-men - yeah, it will be extremely frustrating if the focus becomes race-bending Xavier rather than giving Storm (who's one of the most popular and iconic characters in the franchise) her due.
And the ginormous success of Miles Morales (and to a lesser extent, Kamala Khan) shows that legacy characters, when done right, can make their mark. Not to mention elevating existing characters into legacy roles, such as Sam Wilson or Jane Foster.