The "this format is almost as good as that one" slope is a slippery one. FF is what, 2.6 times the light gathering area af a (canon) APS-C? Well if the FF is barely better than APS-C with that size advantage, then surely APS-C has an even smaller performance advantage over M4/3 being only 1.4 times larger. And so on and so forth until cellphone sensors are perfect adequate for all purposes.
I think you'll find that most reviewers and users are of the opinion that the difference in image quality between m43 and APS-C is very small indeed except at higher ISOs.
Sure, but where does the "very small" end? If the difference between FF and APS-C is very small, and the difference between APS-C and m4/3 is very small, and the difference between m4/3 and 1" is small, is the difference between FF and 1" some degree of small?
Maybe it is, but without quantifying what "small" is, it's a bit of a useless comparison, and in a world where people report for example dynamic range in tenth-stop precision, maybe small from the general lexicon doesn't apply.
You're right, of course, that as a general proposition it's useless - for one thing, even assuming the differences can be measured, what's "small" for me mightn't be for you, and vice versa, and the only way to know is to use the different formats in question and see if you notice any differences that matter to you given the uses to which you put them (or find useful comparisons online). I'm not sure, though, what the point is you're trying to make with your invocation of slippery slopes.