I think the general-interest SF con is kind of a victim of its own cultural success. In the world that was a few decades ago, if you were inclined to go spend a weekend going to an event somewhere in the wide universe of fannish interests, there weren't many choices. Even if you were only especially interested in one facet of fannish interest, you would likely choose to go to a general interest con that included some of that interest, because it was the only game in town. These days, whatever that interest, there's probably dozens of events a year targeted just at that specific interest, and those events are likely to absorb all of your available weekends/money/time, leaving you unlikely to go to a general interest con.

Fundamentally, the reason I think us old farts like to keep going to general purpose cons is that our friends are likely to be there, and what we really want is to be able to hang out with our friends. But people who don't already know the crowd don't have any reason to choose a general interest con when they have plenty of choices of events that more closely match their interest, where they'll be more likely to form connections and then want to go to those events to be with their friends.

In short, I think the decline in general-interest SF cons is mostly due to what's happening in the culture in general, not what the cons specifically are doing.