Champions should only be playable in ranked when you've earned the right to by demonstrating you actually know how to play the damn thing.
Let's assume that reaching Mastery 5 on a champion takes a minimum of 30 games. This is assuming every win you get is an S and you have about a 70% winrate (which is already incredibly unrealistic.) Let's also assume that the average player can play about 4 games per day after work or school.
30 games with an average time of 25 minutes is 12 hours and 30 minutes in total. For one champion. With 4 games played per day and an incredibly unlikely winrate of 75%, it'd take you about a week to get a champion to mastery 5.
##For one champion. One whole week.
Now we're not finished yet, because to queue up for Ranked you need a minimum of 20 champions. Times that by 1 week and you're looking at a minimum of nearly half a year just to be able to queue up for Ranked. That's absolutely ridiculous and completely unrealistic to ask for in a free-to-play game like League.
But wait! There's more! Not everyone is completely flexible in all 5 roles, so you're going to want to have another 3-7+ picks for your main role. Add another 7+ weeks to the count from before.
That's not even mentioning the fact that different people main different roles and a support main is much less likely to get M5 on Darius than someone like Leona, the fact that OTPs exist, and the fact that this would be incredibly unfun for the large majority of people. You play your main for a reason, nobody wants to spend 20+ weeks playing champions they don't enjoy just to test their skills in Ranked.