TL;DR Casual Mode sucks, Quickplay sucked a bit less, Valve dropped the ball.
I agree with him that Casual mode kind of ruined the sense of community where people could hop in and out. But I do think he's looking back with rose-tinted glasses a little bit. Quickplay wasn't exactly newbie-friendly and its 'wild west' nature meant that your experiences could vary wildly. Can you imagine the current day bots with access to sprays? Although at least with quickplay, you weren't stuck if the server wasn't your thing; you could just leave.
Casuals not great, but Quickplay wasn't either - people just point at everything in and around MYM as the reason TF2 as they remember it died, when that wasn't the case. Valve's failure came with not developing Casual further and making finding alternative ways of playing easier (Community Servers). The server browser is ancient and really needs a rework.
In my view Casual was supposed to bring a 'standard experience'. "You playing the game for the first time? Here's the basic way to play TF2". From there, people wanting something different could look towards community servers. The intent is fine, but the execution was bad and no attempts at fixing the issues it created were made.
No one thing from an update 'killed' this game, its just that MYM was a turning point where it was clear that Valve as a whole (not the TF Team) stopped giving a shit.