I'm a little wary about thinking 'it was beter in my day', as every generation says that. But I actually think it was in many ways, and this applied to boxing events. I'm not sure whether it's just gotten old on me, but I seem to remember there being a stake in most undercard fights which made them interesting, as well as at least 2 undercard fights which were unmissable. These days I find I often get excited about a big fight but completely lose interest in the entire undercard. Not sure if this is the lack of characters, if it really is just terrible match ups or I'm just getting old.
Pacquiao-Thurman has a great undercard: Yordenis Ugas Vs. Omar Figueroa Sergey Lipinets Vs. John Molina Jr. Luis Nery Vs. Juan Carlos Payano Also, you're old as hell.
https://www.boxingforum24.com/threads/stacked-the-greatest-fight-cards.608104/ Yes, they've gotten weaker.
You are right. Some of Top Rank's undercards were absolutely smoking a decade ago. For example Jorge Arce vs. Hussein Hussein (unbelievable fight) on the undercard of Morales-Pac I. Castillo vs. Corrales 2 had a pretty stacked undercard too, if I remember right, as did Barrera vs. Juarez 2. Abner Mares vs. Daniel PDL was an HBO Championship caliber headliner that was on the PBF vs. Robert Guerrero undercard
Lost opportunities: you get a huge audience that includes a lot of casuals for something like May-Pac or May-Conor and instead of featuring, say, Wilder is a defense against a Stiverne or something and exposing people to a spectacular knockout they just go cheap. Nothing gained to take forward. Not a good business model, all short-term.