It's not a matter of "mistreatment". Riot tried to remove the issue of people fighting over their respective roles by creating a system that matched you with people from other roles. It's not mistreatment as much as it is misplaced idealism. It expects that there's a perfectly or near perfectly even amount of players wanting to play every role in every elo and division. And... well, it's idealistic. There's no shortage of players overall (unlike Dominion), but reality has shown that there's such a massive disparity in numbers, if you aren't a support, you're probably going to have an extraordinarily long wait time. If you ARE a support, you'll find a match in less than a second.
It goes without saying that there's a problem when one group of players have such a grossly disproportionate wait time than another, but what the hell is Riot supposed to do? Are they supposed to go back to an older, more inefficient system where people will throw down the ultimatum of "Yasuo mid only or feed" and potentially threaten your games? I know from personal experience that it just being ranked or a person supposedly being from a higher division won't stop players from being a massive shitheel made of permaban material just because they didn't get the role they wanted.
Riot's new Dynamic Queue is a more efficient system on paper and partially in practice, but the matchmaking only works in an idealistic environment because of the player diversity, or lack thereof. Introducing Solo Queue might make the issue worse, but the old system was not only uglier, but it was also more inefficient, and you had to deal with the ultimatums like I previously mentioned. There is no ideal outcome; Riot, and us by extension, are damned in every circumstance.