I seriously wonder how many people perused this book and thought "SL" was there as the author's initials!

Understood, and I tried to word my comment not to intimate otherwise. I happen to come from a cryptography perspective, where "if A is easy, then factoring is easy" is actually a commitment that A is in fact hard.

In response to the final paragraph here, I interpreted the original statement given by the "guru" not as a suggestion that there would be a "dramatic simplification" to the overall list of proofs, but rather that showing directly simple groups are 2-generated was likely to be as hard as the full proof of CFSG. Meaning: I suspect the guru is in complete agreement with this post (as indeed I myself am).

Had the proof been included originally, it is not clear the subsequent exchanges would have occurred. Indeed, it's not clear the first paper would've been published, akin to LSpice's comment at the start of this thread.

As I recall, interested people asked him offline for the general shape of the proof. This resulted in some back-and-forths that eventually turned into stronger results and co-authorship of subsequent research that included the result as a special case of something more general.

In my previous comment, I forgot to mention... Adding the sentence didn't even add 2 lines to the paper. Including the proof would've added at least 2 pages.

As for another example, my advisor published a paper classifying groups that have Property X as being among 6 examples. He then made a statement along the lines of "in the third case, we can actually show something stronger, that indeed property Y holds." It wasn't needed for the "if and only if" proof, so was extraneous to the paper at hand. It was very extraneous to the particular task at hand, and only some of the audience would care about the additional info. But there were some who were interested. But not ALL were interested.