Individual player skill shouldn't be confused with the ability to discern misbehavior. Just because they're Diamond doesn't make them any better at sussing out what is or isn't good sportsmanship or deliberate trolling than people at Gold, or Silver, or those who are unranked.
Beyond that; the Tribunal was iceboxed because it was horrifically slow with a low participation rate, and wound up being horrifically inaccurate when participation was incentivized via Influence Points. You're suggesting that they add the Tribunal back, but limit it to a very specific, very small subset of players in the community, who have to actually climb to that point as well before the Tribunal can actually come online.
Which, sure, for most Diamond players, it'll probably be child's play ripping their way through to Diamond, but the point remains; there's at least one day of downtime for the Tribunal if it's being tied to something that players have to climb to/unlock. That in and of itself makes it worse - albeit marignally - than the IFS that we have now.
...and the highrer your rank, the more efective the punishment is...
This would be legitimately unfair. Why put greater power in the hands of a small subset of the community just because they can play well? Why have basically everyone else's reports/reviews/whatever matter substantially less just because they, for one reason or another, aren't in Diamond?