You don't have to buy it ... that's your prerogative ... They believed that the environment became toxic and nasty when it reached critical mass and that was driving new people away.
I don't "buy" the toxic environment in the classical "nasty" sense. The were no more complains or insults on country or ch 200 than we have today. In fact, the most "toxic" and unfriendly environments I have ever flown in were the absolute low-number Early War and AvA arenas. If there ever was an "unhealthy" environment & gameplay, it was in the EW a few tours after the split.
What is being different that there is much less social positive interaction possible when numbers are getting too big. In a crowd, a single new person can easily feel being lost & lonely.