Independent Rankings for eatch role

Why not have independent rankings for eatch role?

Like for example as ADC : Gold, Support: Silver. Now becouse that player is a Gold ADC, but not at support, if in one game he gets to play support he will be queued as a silver support player instead of gold.
As for the overall ranking position the higher one should be the one that matters, this way a player can choose to stick to one role and keep climbing with that role or improve in other roles. Regarding placement games, of course there is no sence in doing 10 games for eatch role, so insted after the 10 placement games all roles are assigned with the placement results.

What we have now is one overral ranking, so a player can be Diamond but only plays one specific role, but the other roles he might not play at that same level.

This also can give players some feedback on how they are with specific roles.