I'm curious, what exactly is there to lose from preventing players taking the champion into ranked mere hours after release?
They're just being toxic in doing so. Your team isn't going to find you yelling "Kindred/Ekko/etc" main in the chat the day of release funny, and if you really think about it, you're just deliberately trying to provoke something which is toxic.
What does being able to take new champs only seconds old into ranked offer VS not being able to? What benefit are you so afraid of denying Riot? These aren't well meaning people.
EDIT: There are only 2 types of people who would spam a new release champ in ranked in the first week. Toxic players who just want to provoke their teamates, or players who have found a gamebreaking exploit/bug that wasn't picked up before it hit live (like the GP barrel gold bug).