I don't believe it has a direct correlation with the fall off of heavyweights.There was a SEVERE decline in true marketable stars throughout all of boxing not just Heavyweights
For example in the 80's you had Sugar Ray Leonard,Thomas Hearns,Duran, and Hagler as legit non-hvy weight stars .Not to mention Boom Boom Mancini for cacs and JCC for the Mexican's.
In the 90's it you had Oscar,Sweet Pea,Roy Jones Jr,Hector Camacho,JCC,and Prince Naseem ....
2000's- Oscar,Sugar Shane,Floyd,Manny,Felix Trinidad,Barrera,Morales, Fernando Vargas,Gatti,Zab .....
If you look at boxing now,how many nikkas do you see with STAR potential in non-heavyweight divisions?
Broner ? Had a shot but that nikka loses too much
Canelo ? Cool,but he's Mexican so he has a ceiling.
Spence hasn't been tested yet
GGG? Crawford? Thurman?
The star power just ain't there across the board