It's not a myth, it is actually better. Bigger sensor = bigger pixels, and bigger pixels are better. Not to mention the crop factor! (or lack of a crop factor...). Full frame is full frame... pretty much everything else is less.
That's not to say that full frame is always better in every way. Bigger cameras, more expensive, etc... but there's a reason that pretty much every pro photographer shoots full frame the majority of the time.
People throw around the word crop factor with a really poor comprehension.
Should be enough to debunk the myth you've stated. So this is what happens lenses will allow light through, if you're using an appropriate lens for your model of camera you the focal length you will need needs to be shorter to achieve the same effect of or view range of a full frame censor. The same laws of physics applies to projectors. Only at the same focal length with the same lens does the cropping occur. The idea that it captures an image cropped is illogical. You just can't treat a 55mm lens the same way on an FF you would on a M43rd. 50mm ≠ 50mm full frame vs smaller censor, but it could with 70mm = 50mm. Just an exaggerated example.
Also in other words the second you throw a lens adapter or anything that extends the distance of the lens to the body you forfeit what that lenses' specified distance is anyway. You've just crop factored your FF, just a little bit.
Only reason FF is better and not equal to is when you can buy a 3K+ full frame body in cost assuring it's censor density is high, megapixels themselves do not scale well unless the censor matches it. A poor censor FF will not improve the quality purely because it's a FF. Some other reasons to choose FF over a smaller censor is to get a more shallower photo.
Speedy, you're really grasping at straws.
I understand that a 100mm f5.6 on Full Frame should produce the exact same exposure as a 50mm f2.8 on M4/3, lens equivalence works to a point, but you're fighting tooth and nail every time you want to achieve something like that, the quality standards on that lens have to be twice as good, and as Full Frame lenses get better eventually gathering equivalent detail will be impossible. It's just a losing battle in general terms.
These systems certainly have advantages in certain applications, like heat dissipation and 4K readout, and pixel density in focal length limited shooting, but in general terms you run into a lot of walls pretty quickly.