The big obstacle is time. It's sort of like saying, "What's the big obstacle with changing all of the classes that depend on spells, like the Paladin and the Ranger, to spell-less classes? You can homebrew anything!"
But, really, magic or not needn't require such sweeping mechanical changes. 5e does make a lot of stuff magic (including Monk's Ki powers, for instance), and in a few situations, it really matters, the big/obvious ones being Beholders' central-eye-beams, anti-magic shells, arbitrary anti-magic zones, and, of course, dispel magic. 'Magic' Resistance in 5e, which just affects saves, maybe not so much.
So, if you want to declare that Ki isn't magic, it works in those situations, while magic doesn't. FWIW.
Same goes for psionics, just declare it 'not magic,' and it's not. Yeah, you have the teeny issue that, when you swing the giant rubber mallet that is anti-magic at your out of control high-level campaign, it misses the psionic. But, really, at that point, you're past the expiration date, anyway.
In a very simple way, this rule change would make a Mystic devastatingly effective in a world where everyone else is using magic.
How often does Dispel Magic, et al, come up, anyway? Is it really so much worse than casters in the first place, since most of the world doesn't get to use magic, either?
And it would also mean that many of the published materials (including adventure paths) would be unusable without a lot of work; more importantly, it would be hard to know what even needs to change. All of this because the system wasn't designed from the beginning with the difference.
I'm really not see'n the work needed. Sprinkling some psionic NPCs and monsters onto the setting shouldn't be a big deal, I'm sure we'll be getting some along with the mystic. Just flip the same switch for them, they'll be able to dispel the psionic's stuff, but not the wizards. As 'balanced' as anything else in 5e.
