The 5s queue works like old teambuilder where you find a team of 5 before finding a match. Riot can pitch this as a way to make ranked 5s more accessible to everyone who wants that pure "teamplay" experience. Solo queue is self explanatory. Both queues would be completely separate from each other and fair.
Except that they would increase queue times by dividing the player base. Which is exactly why Riot doesn't want to do that currently, as the only combinations would be (2+3)(and someone would have to be a support in one of these... Good luck with that.) and (5-man premade) since the (2+1+1+1) (4+1)(should not exist) (3+1+1) (2+2+1) teams would not be able to exist without solo players. Without the 2 and 3 man premades it would take longer to fill each position(very little people queue as support alone, for good reason), and without the solo players it would make the matchmaking longer. By a lot.
And something like the old teambuilder? You mean 10-60 minute waits for everything other than support? There was a reason why it was shut down.(Hint hint, it was the one that I just explained)
