what ranked has become

Actually it worked quite well. Individual ranks were reflected accurately because players paired with other random players every game (with the exception of duo's). Now, you might say that having random teammates would alter the accuracy of someones rank, but if you know anything about random sampling, you would know these confounds are creased out with the increase of sample size ie. the amount of games played. The influence of teammates would become less of a factor over time, and the individual skill of the player would eventually become the dominant factor.

What you say was the problem with SQ is amplified in DQ. Individual ranks are never creased out because the playing field is never even. Even if you rarely get paired with/against a premade team, it is bound to happen eventually. Now you have a whole bunch of variables involving communication and coordination that aren't controlled for. With the premade option the team with more people has an unwarranted advantage over players with less people or who are playing alone. Regardless of how 'balanced' riot makes the queue or how many games you play, there are simply too many variables influencing the validity of your individual rank, and so someones individual rank becomes inaccurate, and for the most part, redundant.

So yes, "Individual rank for a team game is inherently not going to work". But then I ask you: Why did Riot make individual rank more of a team game? There is no logic in this and Riot knows it, they only stood by the idea because it hauls in the most bucks. Individual rank in a team ranked queue is nonsensical and completely illogical.