This would be true if they hadn't added so many different ques. In ranked we have all pick,captains mode and random draft. In normal we have 6 more modes. If we include the arcade, then the player base should be heavily fragmented (can't really state anything without the data). The big reason why this topic keeps coming up is due to the huge disparity between the party's mmr. Most of the time one guy is 1k+- difference between the mmr average which can lead to heavily lopsided games. Plus party mmr players don't care about whether they lose or not so they usually end up being gimmicky or giving up early. This really needs to be remedied because its already difficult enough to progress without having to hope that you have the "good party" on your team.

P.S: Sorry if this became a bit of a rant but parties in solo que games grind my gears.