They have actually gone on threads similar to this a few times I think. Basically it goes down to player's choice.
You want to first-time Aurelion Sol in ranked? You can- nothing stops you.
You want to first-time Ashe in ranked? You can- nothing stops you.
It's up to the individual whether they believe they're capable of winning as the champion- not the other players. This doesn't change for new champions vs old ones.
However, on a personal note, for some reason these threads always assume you know nothing about the champion; which I find dishonest.
Riot releases videos and written summaries of what they do.
Youtube is chock-full of people on the PBE showing you what they do.
At that point all you lack is personal playing experience, which is no different from old champions you haven't played either.
I agree you shouldn't first-time stuff in ranked, but I disagree that it's something that needs to be prevented, it's player-choice; not yours.
Unless you ban it- in which case it is indeed your choice.
Which, funnily enough, was one of the points of the new ban system- everyone gets a choice. You don't want someone playing the new champion? Congrats- you get your wish.