If you stop paying (e.g. if they increase prices on you), you lose the ability to publish new content. This is different than "free forever." Their "promise" is that existing content will, to the best of their ability, never disappear from the web.
I'm not sure how this is a response to my comment.
Mandrill's free tier was "12k emails, free, forever". It didn't last forever. The use of "everything in our power" in this announcement makes it essentially meaningless.
