In reply to David.

I've helped a lot of WordPress sites in my country with all sorts of issues - and 75% of them was due to lacking updates. Making this mandatory will cause even more lacking updates, and potentially evolve to a security-issue as well.

The ideas is good - if only everyone would keep their sites updated.
But a notification, that doesn't go away by itself, would be the way to start off at least.

// Aris