Asking everybody to flex is extremely nonviable. When you get a team with 5-6 support mains, which happens WAY more than it should, it’s nearly a certain loss.
Like if you look at ANY sport you rarely have interchangeable roles right? Even in Esports you NEVER really see people in roles flexing in competitive play.
For example you don’t have many goalkeepers in football/soccer playing a forward position randomly? Or in League of Legends you don’t get your support to randomly change to top lane or jungle mid tournament?
Some players like to specialise in a certain role for whatever reasons they have, they might not find other roles enjoyable. Why force it so hard?
Role-select NEEDS to happen, for the sake of a fair/balanced competitive environment.