Usually whenever I face DuoQ's the matches are blowout 510-36 matches. Most of the players on my team can never get out of spawn and it turns into a ragefest, people give up and it's a free win for the team with a DuoQ. DuoQ's often profession stack, trade cooldowns ect in a very organized fashion making it incredibly difficult for random (but mechanically sound) players to get any sort of leverage in a fight, even if they play well and land critical combos that would otherwise secure fights vs uncoordinated SoloQ'ers.
In contrast, matches where there are no duo q's usually are very good 500-350+ games. You can clearly see/feel what team-fight/1v1 wins led to the upper hand and eventual match win. DuoQ is just whipe mid, trade kiting cd's and then push far and farm on respawn. That just wouldn't happen that often in a SoloQ only environment. As evidence to just how effective and easy wins are as DuoQ, if you look at the rating of everyone on the leaderboard, most of the DuoQ players have bubbled up to the top while the SoloQ only players struggle to push past Plat 2. Plat 1 and Plat 2+ are where all of the decent DuoQ's sit.
When a new player comes to the game and gets pulled into matches where they constantly get farmed out of respawn, they probably won't want to log in again. At this point I think the problems that DuoQ causes outweigh the negatives of having SoloQ only. Both for the health of the game and the players.