The thing with Burnham (and Discovery in general) is that the character seems to fall into the “love it or hate it” category whereas Kirk and Spock are universally liked or at the minimum, respected in the broad sense, I think they have more of a mass appeal for a variety of reasons (better character development, story writing, relationships, longer history and impact in Trek reaching across multiple generations, etc).
I’ve never come across anyone who has said they disliked Kirk or Spock, Mike Burnham on the other hand…
I think anything from the TOS and TNG eras would naturally outsell anything from the modern era IMO.