How is open queue a side mode of the game? It is not in the arcade or anything. It is featured right on the QP and Comp screens, with an icon for it that’s just as big as the role queue icon. The reason it’s not considered the main mode of the game is because, despite open queue being featured just as much by the game’s UI, people greatly prefer role queue and therefore it is the most popular game mode. So your argument is tautological. You say open queue has fewer players because it’s not the main mode of the game, but the reason it’s not the main mode of the game is because it has fewer players! The reality is just that people prefer role queue.
And how did you not have the 5 DPS 1 healer games? Did you ever play QP? Did you always just play in large stacks? The only way to maybe avoid those games was to either always play in a large stack that had people who would play tank and support, or to be really high ranked (like, basically just GM) and only play comp. Maybe you were in one of those groups. But, even if you were, can you conceptualize that what is better overall may not be entirely dependent on what’s better for you personally? If you didn’t experience a massive issue that the vast majority of the player base did experience, then that doesn’t mean that wasn’t a highly problematic issue.