The act of you linking the video in this context is very ironic. Yeah, I wonder why girls are less topless nowadays -- maybe because the chances of their private parts ending up on YouTube is much higher?
My father is from an Eastern European country so I traveled there as a kid. The cultures were very different. You saw bare breasts at pools, in tv commercials, etc. Few people owned cars, no one wore deodorant, all of the food was local style, ie people didn't even eat pasta. It was truly like stepping into a different world.
Now with the internet it is basically like the US with a different language. Being nude is frowned upon in the US and it is now frowned upon there. No one is going topless to the pool anymore and you can find pasta everywhere.
If it's normal and there's no social stigma, then who cares. But now if there is increasing stigma, at least in that the images could be used against you in the future, then that seems more like the problem.
Warning, this is analysis from not an anthropologist/sociologist: Maybe the prevalance of porn and screens to view them on has also made for creepier men/men who are now more used to viewing women as objects. I remember reading an article many years ago about how teenage girls are acting more like porn stars because they believe that's what attracts guys...
As someone who grew up as a teen without a phone, I can say that mental pictures can be enjoyed (even without ogling), and it was very common.
I agree that the second paragraph is likely. Although if the girls are acting more like pornstars, wouldn't that make nudity less of an issue (or more normalized) even with a sexualizing audience?