That's not what he's saying at all. He's saying if Ranked stresses you out, makes you angry, then to simply play normals. If normals stress you out because you lose, then yes, PLAY BOTS. There are bot only players, actually. Not that it's my kind of thing though.
You know, he's not asking people to quit the game, he's asking them to play the game for enjoyment, not for a shiny ranking to show off.
I love ranked, but what he's saying is nothing comparable to your slippery slope fallacy.
Also, there are a lot of normal only experienced players to. You have no idea because you largely play with ranked friends, and browse things like the lol boards, or lol subreddit, which are filled with competitive like minded players that like to discuss balance changes, meaning they want the game more balanced so, in turn, it's more competitive.