So... what sort of "pile up" are we talking about? A massive influx of users all at once? Or a slow, steady trickle?
A slow, steady trickle means that you slowly filter out objectionable people one at a time, every other day, every other week. It becomes a non-issue.
A massive influx of users all at once?
...well, here's a little secret. Wanna see the most controversial subject of the last Reddit influx?
We have it now. We have the ability to filter users newer than two days. Go try it on your own page - it's in preferences. Look at your front page, then turn on the filter, then look at your front page again. Is that censorship? Since the only people experiencing the change is you, it's choice.
Let's be honest: I'm a default mod. It's worthwhile for me to follow #reddit. Right now, however, it's not worthwhile for me to follow most of what's being posted to #reddit because it's a bunch of new guys going "hey, are you from Reddit, too? Wow! So am I! How bout that fatpeoplehate thing?" and it effectively drowns out everything else in my feed. So I flip the switch and all of a sudden, my feed is useful again. Chances are good it'll look much the same in two days as it does now, because I doubt we're going to keep getting an influx. And if we do, I can ignore #reddit for a little while and it'll be fine.
Note this, too - I'm interacting with the newbies plenty. I'm just choosing to interact with them in places that interest me, rather than places that interest them. This is beneficial to both of us, 'cuz I'm not tripping over everyone and they're not suffering my crankiness. Come Monday we'll all be friends.
Can I envision a scenario where functionality collapses? I don't have to. It collapsed last time. This time we have a fix. Is it a forever fix? Probably not. But we're all pretty sensitive about instituting bomb-proof architecture if we aren't expecting The Blitz as it tends to be ugly, unfriendly and cramped.
The really interesting thing is that under the current system, you could have one group of people follow #animalrights and filter everyone sympathetic to PETA, while another group of people follows #animalrights and filters everyone sympathetic to the NRA. In the end, you'd end up with two distinct communities, both interested in a subject, but rarely interacting with each other.
That, to me, is the genius of the system.