In 2011, Tumblr was down for 42 hours.
In 2011, Heroku was down for 76 hours in one incident alone.
And that's all before you get to the hubris of the assumption that your custom coded setup will perform better. A more sensible argument would be to move to a host with a better proven uptime record, such as Blogger (nearly 100%).
Sources: http://allthingsd.com/20111215/tumblr-had-42-hours-of-downti... https://status.heroku.com/incident/151