MLG Columbus had a HUGE peak viewer peak due to the first NA team in finals ever. The average viewership numbers were higher in Boston. I remember the last 2-3 years of majors pretty well, and MLG Columbus had like 150k-200k consistent on main channel twitch, on quarters/semis around 300k-400k. For Boston, the numbers were consistently 250k-300k on groups and the semis/quarters 400k-500k.
But where the real change is, is that it's more global. The site esc.watch records numbers from twitch (all official channels), the chinese site with the rights and also youtube. The peak numbers ARE increasing, and chinese viewers bring in just as much money, if not more, than western ones due to how big esports is in China. So yeah, the game isn't dying, Valve is just focusing on the Global market and by doing so, the peak numbers since Atlanta 2017 have increased drastically. He may say Columbus had the peak viewership, but that's wrong, they had the peak twitch + in-game viewership, but that's not counting the global market for CS which was tiny at the time, and has grown since start of 2017.