The only thing that would likely happen in both cases, craigslist and email, is people would set up free alternatives instantly. Everything needed for that is out there for free already, ready to run.

I agree the free alternatives would be set up, but in the case of Craigslist I think it probably has enough critical mass to keep its dominant position.

A free email provider would still let you send and receive messages from anyone. A free classified listings site doesn't give you access to traffic on the paid classified listings site.