Which can't always happen even in the current champ select because you have to pick two roles, meaning you can't always face people at their absolute best.
The whole MMR system is designed to eventually seat you to where you play with people who are as good as you, meaning both your lows and highs even out to a certain extent. The only way to guarantee your role every time you play Ranked is... surprise surprise, playing in Ranked 5's.
I'm not saying your line of thinking is wrong, but at the same time it's an opinion. For some it might not be enough to have two roles, some would believe you should be able to choose just one role in return for longer queue times because, as you said: when people play their best role you see more of what they can bring to the table.
For others, the fact that you can pick your role is counter to what solo queue was all about, back when you could not guarantee your role and you sometimes got put into a role that you were absolute pants with. Some people would believe that your rank should be reminiscent of how well you could scale the ranked ladder in a system where you were sometimes put at a major disadvantage and still have to find a means to win.