Dynamic Q is better for Riot as they're trying to shift League to become a community game. You know, like WoW. Honestly, like half of WoW players still play because their friends are playing. Riot has too many problems to fix to make this game better for solo players, so they decided to push out a blanket "solution" that would deal with toxicity, abuse, boosting, scripting, feeding and so on.
As for coexistence, Dynamic Queue sucks peanuts for literally every soloq player. it does not give any benefits to solos, only drawbacks. A lot of players don't like supporting because it is very dependent on teams performance... guess what happens when you play with a trio or a bigger premade. It also makes the game a "premade roulette" because if your three-man etc is worse than theirs, you have almost no chances to turn the game as a solo. Facing premades as a solo also seems quite unfair as they can coordinate ganks/tps/roams and the solo player can only hope that his teammtes are watching and will answer his calls... which almost never happens.
I imagine no solo player would ever play dynamic if soloq came out, just due to sheer reason that they would feel (and actually be) more important in a pure soloq game.