Why is CSGO dying

Personally, I'd argue that it's not dying. It seems like people are jumping to conclusions since PUBG has been such a massive success and many CS players (specifically streamers) have been playing a lot of that.

What's important to remember is that CStrike is the only truly competitive FPS out there. The only other FPS games played as E-sports are Overwatch and CoD and those are nowhere CLOSE to the size audience CS has. Prize pools are higher in CS (and still increasing).

The main issue with the game itself is just that Valve's updates, however frequent they may be, disappoint people because of their lack of significant content. It seems like every Tuesday update we get is just filler and bug fixes and ends up being 300MB anyway.