Is this measured by number of online games played (e.g. using lichess data), or by number of spectators for competitions in the respective variants?

I think there's a chance but most likely classical chess will still be #1. There's a lot of very good young stars rising in Classical chess, one of them will probably be the new face of chess. I think there's resistance to calling it Freestyle chess instead of Fischer Random from some GMs as well.

@AnthonyPeterson For market clarity, whether its called Freestyle / Chess960 / Fischer Random will not matter. What matters is whether the game format takes domincance.