I beg your pardon but this is exactly how it works.
The full quote is: "There is no rational reason for wanting a psion that shouldn't be addressed by better design of the core magic system "
If there isn't a rational reason for wanting a psion, then there is no need to change the design of the core magic system. If on the other hand, there is some rational complaint about the core magic system then that complaint can be addressed by refinement of the core magic system.
I'm not the one asserting that the magic system needs a better core design or even that it needs to include psionics at all. In fact, I've made pretty clear that I think the game is better without them. But again, if there is a rational reason for wanting psionics because of some deficiency in the core system, it's better addressed by refining the core system.
And further, even though I'm not under any obligation here since I'm not the one dissatisfied with the present system, I've already speculatively engaged in this process. For example, some commentators on the psion express joy that they have more "fine control" over a spell in various ways by being able to inject points into the spell to spontaneously metamagic the spell. And that seems like a potentially reasonable non-selfish and thoughtful complaint given how awesome the concept of metamagic is and how generally unsatisfactory the implementation was in 3e. But as I pointed out, a cool solution to that problem has existed since way back in 3e when Monte Cook in reimagining the D&D magic system for his Arcana Unearthed project built metamagic into the spell descriptions as a universal property of the system. And I'd also like to point out how the system he built is in a lot of ways congruent with the 5e Mystic class where it's "spells" could be cast at multiple power levels depending on how many points you put into them.
So it seems to me if this complaint is valid enough to justify the psion, then it's valid enough of a complaint against the system that every magic using class ought to benefit from it and not try to solve it by tacking on an extra class and extra magic system. The design work should more properly go into refining the core system.
I think what I said was quite clear.
