As one of those players, I stopped when I finally realized ranked in league wasn't actually competitive because Riot had no interest in making it competitive.

And because I no longer play I have, over time, completely lost interest in watching it. Part of that is that the OG players I used to watch have pretty much all retired save a select few.

I'm sure there are people that continue watching it long-term after they quit playing, but I doubt it's a hugely large percentage. The only exception is going to be championship games, where I'll still sometimes tune in (especially worlds) until NA falls out and then I don't even bother watching the actual final game.