The absolute traffic numbers immediately might not drop that big but the power function of participation - number of users is brutal for Reddit. There's a tiny, tiny fraction of users who were to leave the entire structure would fracture. The house doesn't collapse immediately but it's unsound so people begin to leave. First those in the know, then those who follow those in the know, and so on larger and larger waves until only a few people roam the ruins. It might just go on and on -- digg.com is still up. Even Slashdot is still there. Heck, I use lycos.com to force the login screen when logging in on public Internet.

This is the problem here: these people already started to delete their content and will withdraw their tools in three weeks. Time is up and the CEO doesn't even acknowledge the massive problems this change causes for moderation tools. His post should've been an about face but no.

So, no it won't be 20-30% immediately, might be so small it doesn't even register beyond normal day to day variance. But it will be 90%+ in a year.