So, a diamond player is diamond, no matter what. They can swap roles, play new champs, all that -- but they're still diamond. They have a better understanding of the game, the champs, and all those little interactions you learn over time playing the game. A diamond ADC main who is playing mid for the first time ever will still dramatically outperform a silver mid main (assuming that diamond actually deserves their rank). Despite that, they could still lose because their bot lane decides to feed, and despite everything that diamond player does, they're unable to stop the snowball. That's got nothing to do with the diamond player in an offrole.
**The issue is not people playing offroles and doing worse, it's that the ranked system does nothing to recognize each individual player -- it's all about the outcome of the game, which is determined by 9 other players who are out of your control.**
######(Moreso these days than previous seasons, with how fucking absurd damage is)
If you want to make ranked fair for the individual, don't just cover up the issue like you're doing. Retool the ranked system to modify LP gains / losses based on your ***individual*** performance (not per game, but over an average that builds with every game you play), based on stats that are important to the role you play.
For example: A diamond player who was boosted feeds every game, and can't function in high ELO. But because they were carried by a booster, they got a rank they don't deserve, since ELO is based on nothing but the outcome of the game.
Now, with a proper ranked system, that person who paid for a boost is performing horribly every game, and so their LP gain is penalized, while the booster gains LP faster, eventually making the rank disparity too much, meaning the boostee can't be boosted anymore -- forcing them to either pay a new booster, or just give up and stay in their own rank.
A second example: A silver player who plays far better than most people up to say high plat can't climb because of bad teammates. With a system based on individual performance, even if the silver's teams are bad, if their performance is consistently higher, then they'll eventually climb to where they belong, since their LP losses are minimized, while they gain quite a bit more on winning.
If you wanna talk about fair matching, that seems pretty fair to me. But it doesn't artificially extend the average playtime of the population, so it doesn't have any kind of money making potential, and I know it won't happen.
**TL;DR: Make a ranked system based on individual performance, and all the complaints go away.**