I don’t think Haymon leaving HBO caused a popularity decline but imo he could have done things better later on.
Imo the main problems were…
The over reliance on in-house fights while probably thinking that they can make others on the market irrelevant. It never happened and it just looked bad.
Letting some of his fighters to have long inactive periods.
And finally that he didn’t invest in top talent the way some of his rivals did.
That’s not to deny from PBC that they really built up some stars over the years and there’s still hope for them for the future but now they are in need of some blue-chip talents and a more consistent schedule.
The issue is people thing Boxing is more important to these media companies than it really is. With the rise of streaming and companies going deep in debt boxing became the easiest fat to trim.