Well, I think a huge addition to not just Soul Calibur, but fighting games in general, came later in Online play functionality.
Fighting games are one of those genres that would probably sell a lot less than they already do nowadays, if there was no way to play vs other people online. Because they're generally considered difficult to learn. And for those who grew up with them since the beginning they probably don't have the same time and opportunity to get a couple of their friends together in the same room in person, like they used to back when they were kids.
Suikoden series. The second was such an amazing game that the 3rd seem bad in comparison (even through I think it actually was a decent game but a huge drop off from the second). The 4th one was just bad don't think I ever finished it. Never played the 5th.
I feel pretty much the same way. They tried some new things in 3 that were hit or miss. I liked being able to play from the perspective of different main characters. Especially as the badguys. But the friendly fire and support system in combat was wonky.
Although I really liked 5, as it reminded me a lot of 2, and it's my second favorite in the series.
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.