DuoQ was blocked for some time 2,3 years ago and it was great. The soloQ experience got better and players overall got better. But pro players complained a bit since they can't practice, so Riot allowed it back.
With the introduction of new server on NA where pro players will only play against each other and can practice as much as they want my guess is that one day they will remove DuoQ again. SoloQ should be for SOLOQ and in order to be better and improve so you can one day go pro, you need to play alone and show your true skill, leadership etc.
Playing duo with someone ruins the MMR, ruins the competitiveness, ruins overall experience. Getting your duo to camp you entire game so you can snowball and win without any game knowledge, just brute 1v2 force, can win you a game and it's disgusting.