Online functionality was the main reason I bought Soul Calibur 6 so that's true enough, but I was more thinking of improvements to the gameplay itself. Dunno if you're right about sales of fighting games though. In my younger days, I was content to play Soul Calibur for hours a day just against the computers. I'd imagine more people then you'd think still buy fighting games just to do that or play local multiplayer.
Edit: Also though Soul Calibur 2 does have a version with added online play. Was tempted to go for that, but I figured the userbase would be bigger on SC6 and even with that it takes longer to find a match than I'd like.
I also liked to do that when I was younger. Though it wasn't necessarily my age that changed this, but because I got more into playing them competitively.
I don't know that it's more than I think that still do that, but rather that fighting games are generally pretty niche, with a few exceptions. Mortal Kombat for example has a very well made story mode. And Dragonball FighterZ is also a game that managed to be more appealing than usual to a non-competitive crowd, which can be seen in the 90% drop it had in its online userbase from week 1 to week 2. It started off at around 42 000 on Steam, which was a record for a fighting game on the platform, and then quickly went down to the normal 1k-2k that they usually have in their first few months on the market.
But I doubt there would be nearly as many entries for games like Blaz Blue, Guilty Gear, King of Fighters, etc, if there was no online component, because for a lot of them that aren't named Street Fighter or Tekken, they probably wouldn't be profitable enough without the competitive playerbase.