Answer

I don’t think it’s some brilliant flash of insight, but it definitely seems to me a reasonable and correct explanation. The Mishnah, and Maimonides following it, are presenting here the law of proximity. The novelty is that the rule of proximity applies also in the areas of prohibition and permission and in monetary law. They did not get into practical questions here: what happens if one side has a majority, and what if the coin resembles those in one pile and differs from those in the other pile (for example, perhaps one is shekels and the other dollars), and so on. That is the way of the Talmud, and from here comes the need for limiting interpretations. I do not see this as any real difficulty. Incidentally, in light of what I explained in my article about limiting interpretations, you can understand that even someone who explains the case as involving equal piles does not really mean to say that this is what the Mishnah was actually discussing. His point is that the Mishnah comes to introduce the rule of proximity. And when you ask, “But what about majority?” a limiting interpretation will be made that it is speaking of equal piles. But the novelty is always true, even when the piles are not equal: namely, that the rule of proximity is relevant in these situations as well. See my article there.