People dislike BfA due to lack of current content, dislike meta directions (like azerite), and dilution of the sense of community. Classic addresses the latter 2 because, for a lot of people, they are the most important. People aren’t playing Classic for content.
What they are playing it for is:
- Visible character growth (lost when you can get through 5 of the next 10 character levels without needing a new piece of gear)
- The requirement to work with others, consistently, and to positively interact with them to get future groups. (lost with looking for groups and auto ques)
- Content where you can (and regularly) lose that isn’t -just- mythic raiding. (Not sure when this was lost but I would guess Cata onward)
- The trip to the end content is as important as the end content itself. (lost when level pacing went from weeks to hours)
Hopefully they take these lessons and implement them again.