Both of you exaggerate.
League of Legends isn't dying, but judging from Esports viewership, streamers quitting the game, a lot more people discontented by content than hyped for it, the game has probably hit it's plateau. It's likely to decline somewhat, game is over 8 years old.
He's also not wrong Riot probably expects some banned players to continue on new accounts, likely to spend a little to boost up faster. Even of they don't, odds are good that player will still view enough content to help statistically be apart of the player base that keeps LoL sponsors coming and paying money.
I doubt Riot is desperate to ban people and that this guy believes the Earth is flat.