Love the idea.
However, what happens if the anonymous email forwarding service disappears (e.g. shuts down)? Does it mean users won't be able to access any online service they signed up for with an anonymous email address?
The best way to mitigate against the risk of a service shutting down is to use your own domain name. That way, by simply changing your MX records to another email provider, you can continue to receive emails as normal.