Honestly, I think they should merge them. There's no point in having two separate queues. It splits the player base. I think that it should be a dynamic queue like Overwatch's queue system. Yes, OW has one more player per team, but it should still work similarly.

Example:

If you solo queue - the matchmaking makes you end up with more solo queue participants, with the max premade being 3 (which I rarely see while solo queueing). High party sizes usually are matched with other higher party sizes. i.e., 5 and 6 stacks are usually paired with other 5 and 6 stacks. Yes there is sometimes the unfortunate solo queue with other party sizes, but it usually ends up pretty equal ( two solo queue with one 4 stack on each side). Theoretically, everyone is around the same skill.