Honestly? Attendance rates dropped off a bit, yes.
However, in the past few years, this due to:
Other conventions like the much older Otakon occur rather close to each other, which forces potential attendees into a difficult choice. This then leads into...
Hotels, if not conventions themselves are expensive. Travel is too, depending on where you come from.
-Guests of Honor
I see your point, but frankly, most of the VAs don't just do Pony. Especially Tara. Each VA has contractual conditions for appearances at conventions (ie. cost per autograph, panel limitations, Non-Disclosure Agreements for incoming content, Pony or Non-Pony), as well as their own hectic schedules to balance. The fact they can fit in a convention is amazing in and of itself. I might also mention that these appearances aren't as fun as we may think it is. Perhaps not quite being #HorseFamous, but something pretty dang close.