Everyone keeps coming at this from the perspective that this sucks because you can no longer leave matches in which you can already see that you'll probably lose from the start. You all keep ignoring the up to 5 people that remain, now one(or more) man down, standing an even smaller chance at winning but maybe not wanting to leave/not even knowing you can (so far) without punishment. They will most likely get trashed even worse than if they had you on their team, trying your best.
You also ignore all the matches that don't have a six stack present (which I assume to be the larger portion of all matches), where people still leave for some reason. Matches that may have been fairly balanced and are then suddenly becoming unbalanced because one team has less players.
By leaving matches early, you're throwing the rest of your team under the bus. That rest of the team is gonna have an even worse time losing than when just losing to a team they can clearly see as being the better, more experienced team, because now not only are they outmatched in skill, but also in numbers.
This meta of "oh I'll just leave the match if it's going bad" has been here since beta and this change is finally starting to put a stop to it.
While it's not great that it comes at the expense of those trying to play competitively, this change is the right move for the largest part of the community playing online right now.
The final solution will be the release of server files and skill-based matchmaking, and I'm looking forward to both of those just as much as you.