how many people would even know who Richard Spencer is if he wasn't assaulted in the street by an Antifa thug?
Probably fewer, but he's been a notable figure for a long time. His Wikipedia page dates back to 2013, and it started receiving significant edit attention around 2015.
Not only that, but if you look on Google Trends, you'll notice he gained a lot of publicity back in November 2016, reaching roughly 72% of the Google search traffic he got when he was punched in the face.
I don't doubt that getting punched expanded his platform, but even if he never existed we'd still have a similar kind of douchebag in his place. Organizations of any kind will choose a figurehead.