To my grandparents / great grandparents generation at the time (70/80's) TV has become the world's nanny would have generated all the same comments as this. If you watched tv - the idiotbox - for more than 15 minutes your eyes would go square.
What if there was some truth to that, and YouTube was 10x worse?
For it to be the case I would expect to see a split in the population on many metrics, especially around education, dividing the with / without TV / YouTube groups.
Presumably the adverse effects would overcome the dominant factor which I assume is socioeconomic.
As for the making your eyes go square - I doubt youtube has any more effect that the old tube tv! Its also believed that reading books gives you bad eyesight ...