I kinda have to agree with the op though for numerous reasons. It's easy but very tedious, laggy and annoying to look up people's ranks during matches, it would make gameplay much more fluid if that info was simply in-game and it would make the loading screen way less boring. There's no real reason why Riot shouldn't just provide them for you especially when the coding for it is already there, they just need to activate it, and if anything it would help your team prioritize who who to target early on the enemy team.
If someone is the kind of person who would complain about a diamond in a lower elo match, then they're just going to feed anyway and then say "hey wait, turns out this guy that's 10/0 is diamond omg that's why I'm feeding" They were doomed to give up before the match even began, they will give up over anything. Other players will want to beat the diamond to feel good about themselves.
On top of all of that, if someone simply has a diamond icon, then everyone from the enemy team already knows they're diamond material anyway, but Riot doesn't remove diamond icons.