I believe [Schur's Lemma](https://en.wikipedia.org/wiki/Schur%27s_lemma) was originally considered difficult (after all it did get named). However it is now a one-line proof in an undergraduate course.

I suspect Schur was interested in finite dimensional representations of finite dimensional algebras over the complex numbers. Then the lemma is that the endomorphism ring of an irreducible representation is the complex numbers. Don't ask me why this was considered difficult. The definition of an abstract algebra was not published until after Molien and Wedderburn's results so I can see the statement would have been convoluted.