cool question. From what I know, it seemed as though no one at that time truly thought the barrier could be broken so the legacy in my mind is that the potential for human performance should not be underestimated.
The reason I posted that article is because I found the whole topic interesting. Sub-4:00 doesn’t seem like that big of a deal now, even though there are relatively few runners capable of even thinking about it.
As for what I believe his legacy is, just what the article and you both referenced. He broke the glass ceiling. He proved that what once was thought to be impossible was in fact possible. Once that was done, you saw a relative flood of sub-4 milers. It was now known to not be impossible so people believed they could do it. If you don’t believe you can do it, you can’t do it.