You said it urself, ranked roles rids you of bad experience, whereas your opponent still has to deal with that, hence higher win rate and mmr boost.

Huh? You don't get matched up against a random set of five pubs in ranked roles. You get another stack of people that also queued rank roles. There is no advantage you get over an opponent using that queue.

Dota was just as good when they had that seasonal queue back during whatever Major/International there was year or two ago... The problem isn't giving ranked roles to the entire community. The problem is that ranked roles segregates the player base. It pulls the more serious players, ones who want to win and might let go of their ego in order to do so, out of the general pool and into a special bracket. Players that are like that will climb. Which leaves the general pool more terrible than usual.

Giving everyone access to the ranked roles queue does not fix the problem, it just reverts it back to the old status quo... Fixing the problem means finding a way to put players who want to play seriously and climb in their own pool and let the more casual players, toxic players, players who refuse to do anything other than insta-first pick their favorite core hero and role into their own pool.