Its an interesting proposition, but fundamentally broken. Lets say you build it (and frankly I think a better course here would have been something based on the app.net model rather than WP Engine model) and its "successful" what does that mean to you? Here is the challenge, there are too many scenarios where you don't want to be. #1) "SPLOGGING" or some random thing becomes the new thing which all the cool kids are…
Thanks for this note. It's very much great advice, and right now we're heads down coding. I've never started a foundation and I have much more reading to do -- but it sounds like the right way to codify our pledge more permanently.
Another approach is to build a service where you make no promises about hosting content forever, but find ways to ensure that users own their data, and have full freedom of migration if you should close down.
Since 2010 I've been running http://branchable.com/ , which takes this approach. The same git clone of their site that users can make to edit it contains all their data and configuration. It's built on an open engine http://ikiwiki.info/ , so users can switch away at any time, with no conversion. Ikiwiki is a static site builder, so users can host on S3 or github, or anywhere if they decide to stop using Branchable. Even the management and business logic are open software, so if we folded tomorrow and someone wanted, they could recreate us using http://ikiwiki-hosting.branchable.com/
Branchable is not designed to have a fast burn, but we've had slow and steady growth, and happy users who continue paying the bills each month.
There's a lot of room in this space for sites serving different sets of users in ways that don't turn them into extruded VC product, so welcome to it!