You're right, they're more than welcome to play any mode, I have no problem with that whatsoever and neither has anyone in this thread so I don't know why you keep bringing this up. But if they have no understanding of the game at a competitive level, then go into ranked - which is a competitive mode, hence why competitive mode is on by default - and get destroyed then rage quit, then they shouldn't be playing.
Your entire argument just seems to be, 'keep it best of one so the casuals can get cheesy wins and the opponent has no time to take advantage of adapting to them.' If they're going up the leaderboards due to getting gimmicky wins and thinking they're good, then that's even worse than them losing and learning from it to become better.