The only reason why Classic has gotten any viewership traction is because famous streamers like Asmongold and Sodapoppin are streaming the game. Without these famous streamers, there's hardly anyone actually watching streams of Classic WoW. The numbers are back to where they traditionally have been.
When these famous streamers go back to streaming something other than Classic, their personal viewership will go with them whilst WoW's plummets. So the interest doesn't lie with Classic WoW itself, but with the streamer personalities behind the screen. So any sort of overall interest for Classic WoW based on stream viewerships should be taken with a grain of salt, as these people are there to watch their favourite streamers, not Classic.