I think the decline of boxing is quite complicated. Some of it surely has to do with a decline in racism or at least racial and ethnic identity (but there is still plenty of that as we've seen in the past few years), some of it has to do with a society that isn't as comfortable with violence (but we have the flourishing of MMA and pro wrestling as counterexamples).
But the promotion companies is certainly a huge part of it. In 1960, boxing was together with baseball the premier spectator sport in the US. In the 1960s and 1970s, everything that Clay/Ali did was must see TV, there was a ton of inertial interest, and moving things to PPV didn't change that. The Olympics were huge because of the Cold War and they introduced new up-and-comers like Frazier, Foreman, and Leonard. By connection to Ali, guys like Norton, Spinks (who also had the Olympics going for him), and Holmes became household names. Even if Ali's fights were PPV, guys like Duran, Arguello, and Monzon were on broadcast TV or PPV undercards enough that you knew them.
Let's look at the 1980s. Entering 1980, Ali, Holmes, Leonard, Duran, and perhaps Benitez are household names, and Hearns is getting there. Every single year, you have something that was must see even for casuals:
- 1980 – Leonard vs. Duran 1 & 2, Holmes vs. Ali
- 1981 – Leonard vs. Hearns
- 1982 – Holmes vs. Cooney
- 1983 – Hagler vs. Duran
- 1984 – LA Olympics, Hearns vs. Duran
- 1985 – Hagler vs. Hearns, Spinks vs. Holmes
- 1986 – Spinks vs. Holmes 2, Tyson vs. Berbick
- 1987 – Leonard vs. Hagler, 4 Tyson victories
- 1988 – Tyson vs. Holmes, Tyson vs. Spinks
- 1989 – Hearns vs. Leonard 2, Duran vs. Leonard 3, 2 Tyson fights
- 1990 – Holyfield vs. Douglas
Fast forward to ten years ago. Everything has been on PPV for forty years now. You have only three household names in boxing, Floyd, Manny, and perhaps Wlad (but he is no Ali or Tyson, nor even Holmes on that front). What do we get:
- 2013, Floyd vs. Canelo... this fight makes Canelo a household name even though he loses
- 2015, Floyd vs. Manny is number one, and that fight sucked and was years too late. Fury vs. Wlad, another crap fight, but introduces a guy with a super marketable name to the mainstream.
- 2017, Floyd vs. McGregor, which isn't even a real boxing match, Canelo vs. GGG, which is a rip off.
- 2018, Canelo vs. GGG 2, another rip off.