Well, if I dare explain why I believe Riot can't simply release Solo Queue, I'd get downvoted to hell and back like every time I tried to make such a post.
But well... here I go...
There is two side to the coin. On one side, you have the problems it would help fixing. On the other side, the problems it would add or worsen. (Omg! How dare I say that adding Solo Queue will create problems! +40 downvotes inc)
Riot can't simply make the change because they could end up shooting themselves in the foot by doing it. So instead, they keep it on the back burner while they try to find a solution that would solve the issues a Solo Queue could solve without the need of a Solo Queue.
One change they made, which was a really good one, was that Diamond and up could only queue with people 2 ranks below instead of the entire division. No more Diamond 1 with Plat 5. This is a change that could potentially be seen at lower division, but it may become too restrictive at the same time. For example, I can't play with my Diamond 5 friend in ranked until I reach Plat 2 (I'm Plat 4 right now).
In short, replacing problems with new ones doesn't solve anything, it just means they have to scrap every ideas they had to try to solve the new problems. If they can't find any solutions to the problems a Solo Queue would fix, maybe then they will add Solo Queue and try to fix the new problems instead.
And in any case, as much as I want to agree that Solo Queue would be nice, I simply can't. For the reason mentioned above and really... How can I? People are complaining that they lose because of Dynamic queue when myself, I get a S+ with Nami (And let me tell ya, S+ with Nami is absolutely not easy to get) as a solo queue in a game with 4 man premades on both sides... It's not like as a solo queuer I was powerless to win because there are premades in the game... in fact, I feel like I have a much greater impact in those games because l'm the one who will make or break my team in the end. Maybe it's because I don't play with the mentality that premades are going to ruin the game for me?
So yeah. I can't agree with solo queue.