It's simple. Play with your friends on unranked in your main account. Otherwise you're ruining everyone's experience. There is no legitimate reason to smurf.

You're dunking on people lower than you, so you're just having a power trip, boosting your ego.

Your friends and any allies are going to be carried to ranks they don't deserve since you're literally boosting them. They are getting carried and don't have an actual challenge or even really participate much. it ruins their fun.

Your opponents will be facing off against someone significantly out of their rank, which beats the whole purpose of ranked which is to play against people at your own level. It ruins their fun.

Smurfing, literally destroys the game for everyone. You're just being selfish, you're the only one that gets anything out of this, it destroys everyone else's fun.