>Worse, once a sub is made a default, its quantity goes up but its quality goes down.

This seems to be almost universally true for any online community. As a userbase widens, content quality and even social dynamics become diluted to some extent towards a general population baseline.

As a mod of /r/changemyview (153k subscribers), we have discussed opting out of becoming a default sub if we were approached by the reddit admins. We already continuously struggle to maintain sub quality, given 50k subscriber growth in the last 9 months, though we have seen some success with our small set of very well-defined, strict, and heavily enforced rules.