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I am well aware that が and の are interchangeable in cases like this where there is a "subordinate clause" (forgive my imprecise language). However, as I was working through my N1 grammar reviews, I noticed that が is used in situations (other than the "subordinate clause" pattern) where の could have worked just as fine.

For example,

経済的にあまり恵まれていない地位ゆえに十分な教育を受けられない人がいる。

I think I read somewhere that there is a shift somewhere in history that caused の to take on some of が's functions, but I am not quite sure. Can anyone shed light or share reference on how this happened?