I think you will find that the banning of dance marathons was for more complex reasons than cruelty. Loss of profit by theatre owners, churches considering them lewd etc. Yes one man died, and one woman committed suicide for coming 5th.
Saying "if you don't like boxing fine, don't participate" (and, by implication, shut up about it) is putting words into others mouths, that is your interpretation of it. My interpretation is exactly what it says, don't watch or participate, you are obviously quite at will to object just as others are to be pro or neutral.
I would consider an activity to be cruel if the participants had no choice in the activity ie dog fighting. Barbaric is far more subjective, hence the ban on bare knuckle fighting.
With boxing you either love it, hate it, or couldn't care, any amount of talk is unlikely to change many minds.