My guess is that if Riot was going to reintroduce the Tribunal they'd want to tie it to the replay system so you could actually watch the game in question (to see if they were really intentionally feeding or trolling.) Riot recently has said they have multiple "player behavior" projects going on but none of them are developed enough to announce yet.
I do miss the tribunal though. Some of those chat logs were hilarious. And no I don't think it's a conspiracy for Riot to keep feeders and trolls in the game. Those kind of people wind up hurting Riot's bottom line rather than helping it (since the vast majority of people who play aren't trolls and those people are less likely to spend money on the game if they get trolled game after game.)