I've always wondered about this - I don't disbelieve that some people feel this way, but is it really a general consensus? And is there a reason we should expect it, I don't see why staggered keys make fundamentally less sense.
I would say it is a consensus among people who have used non-staggered layouts for a sustained period - the benefits are definitely noticeable to someone with RSI too.
People are satisfied with their keyboard layout because it's good enough for them, and they've not experienced using other layouts enough to compare the benefits. It's very much like PGs "blub paradox", in which the a typist becomes familiar enough with his keyboard that other, allegedly better keyboards seem like only minor improvements - not worth the effort to switch. For most people, this is probably the case - it's only if you're typing for several hours a day that there will be tangible benefits to hand-optimized layouts.
Even if this is true, it only demonstrates that the original reason for staggered keys is no longer a factor, not that staggered keys are actually worse for humans.
The rest of your post brings up the same questions as the other reply. Even if straight layouts are superior (I'm willing to believe it), by how much, and what about other factors? A typematrix rigidly-square layout is quite different to (for instance) the semi-straight layout of the TECK keyboard marketed as 'truly ergonomic'. In a sense, this layout isn't even different to staggered keys - staggered rows can be relatively straight if your wrists are angled. I suggest this partly in principle - I do agree that the standard angling of a staggered keyboard is not ideal.
A good way to see if a keyboard actually tries to be ergonomic is to see if the left and right hand layouts are relatively symmetrical.
In the context of straight columns, what seems strange about it is that my wrists are naturally at an angle when typing, because the width of my keyboard is less than the width of my shoulders. Straight columns intuitively seem like they would require more wrist distortion, not less. This might not be the case on a wider keyboard, but I don't think typematrix ones are significantly wider than 'normal' keyboards?
I don't know if this reasoning is accurate or not, and I'm sure it can't be a real issue because typematrix keyboards are clearly popular, but I'm not convinced about it just because they're more symmetrical. Plus, in terms of general typing efficiency, I also don't know if I should believe the symmetry is actually important - it might be negligible compared to (for instance) the inefficiencies of qwerty that most people are using, or even compared to simple posture improvements.
The standard layout is just horrible, reaching "v" and "b" with the left index finger while keeping the rest in home position requires an uncomfortable amount of stretching. Even for the right hand, I think the standard layout errs on the side of too much stagger, which is evident when using the pinky to hit the "/" key. There is certainly a lot more wrist distortion than when using the TypeMatrix - there tends to be less distance between the "a" and "l" keys on a standard keyboard as the TypeMatrix places the Enter and Backspace keys in the middle. I guess the Kinesis Advantage might be more ideal in that regard, though I don't suffer from any significant wrist distortion with the TypeMatrix. My hands tend to look like this when using it: https://ploum.net/images/tmhands.jpg
The small size of the TypeMatrix also makes it a win for me, as I occasionally bring it along with the laptop when I sit in the couch for example.
Qwerty is another inefficiency, and I use Colemak myself. The difference between a standard keyboard and the TypeMatrix is still far from negligible.