The playerbase is simply too small for that kind of change, and forcing pre-mades out of automatch entirely would be completely unfair to those players. Even SC2 at its peak mixed pre-mades and randoms.
The only solution I can think of is making the match-maker MUCH more strict when matching a pre-made with randoms, so you don't end up with massive discrepancies in skill that destroy any hope for the random team. Ideally, when searching as a group, your opponent shouldn't be more than 100-200 positions lower than you on the ladder; unfortunately, I get a lot of games where the difference is 500-2000.
It also happens at the top end; even the best 'random' team is going to get destroyed by a decent pre-made. Good coordination is extremely powerful, and can make up for a surprisingly large skill-gap.
I see where you are coming from , but it's not unfair for everyone in a ranked matched to play the roulette wheel on an equal footing rather than some people already knowing what numbers they will get before the spin.. It's actually a lot MORE fair for everyone involved to have an equal footing and then let the ELO do its thing.