One of the gotcha's that needs to be solved is the change in WP 3.9, where some hosts use the mysqli extension for database access.
If there are plugins that directly use mysql functions, and not $wpdb, then they will stop working.

If this proposal is about upgrading Core, but not plugins (which sounds even more impossible), this can be a big problem for some sites. Especially sites that are unmonitored, and haven't seen a login in a few years, they will just break and nobody knows how to fix them.