There is no specific point.
It would be more of a subjective feeling, and noticing that a high proportion of your teammates seem to quit out all the time.
The game is not gonna be fun to play for anyone involved in those groups and people become even more likely to quit out of those groups, because the quitting behavior is no longer be compensated for by other players who just stick around.
It is comparable to impurities and faults in basically any product.
There is an acceptable amount, where it does not really matter, because it is diluted enough.
With quitters, it might be fine to have around 20-30% of them in a pool with 70-80% players who tend to stick around.
But if you separate them compeltely, you will have 70-80% of the players, for which the situation barely changes, while completely ruining the experience for 20-30% of the playerbase.
What would happen if you get matched with absolutely toxic pieces of garbage (who do not tend to quit out), 4 times in a row?
You quit out, because you do not want to have your time wasted and your fun ruined by such filth.
But now you are on the naughtly list for quitting out too many times, so you now get matched with the players who have a habit of quitting out for any reason. Good luck trying to enjoy the game, while getting your quit rate back down.
I do not like dead weight players on my team and i do not like having my time wasted.
I make sure that i am never a dead weight player in any team, by not playing difficulties that are far above my skill level.
I might quit out of a team, if there are a bunch of garbage players in there.
Others quitting out for the same reason, is fine with me.
The players who quit for this reason, are not the problem in this situation. The players who are far too bad to play on the difficulty that they match for, are the problem.
If someone makes a mistake, dies and then quits, that is not the same.
This player is quitting because of their own mistake. I am quitting because someone else made a mistake.
But if that quitting player makes room for someone who is more suited, that is absolutely fine with me.
I do not even think that quitting is the primary issue.
I think that the lack of gatekeeping for higher difficulties is the issue. One of the symptoms is, that there is a high flux of people who are bad, fail and quit.