I don't believe CS:GO is now more watched than ever. Major finals used to have more than 1mio viewers, this year it had 600k more or less if I'm not wrong.
I think it's cuz Valve took of the drop of boxes too but Astralis dominated so much that a lot of people lost interest to watch finals, not only on majors.
YT and FB has always been there giving competition to Twitch, if not always, for a long time now and sinecerely I don't see them as a reason for the decrease of viewers.
Obviously it isn't just Astralis fault, there's a lot more problems to justify this decrease.