How about having some sort of metric to express how out of date an entire WordPress installation is could be given? Perhaps a total of the number of versions behind each active plugin, the current theme and WP core is (with perhaps a higher weighting for core). Inactive plugins would not contribute to the score as generally they pose no issue until activated.

This would be an internal score only, visible on the dashboard and perhaps the updates page, and a higher score would indicate an out-of-date site with a higher chance of security risks.

Seasoned developers and site maintainers perhaps would not get too much from this, but typical hobbyist or content-focussed owners that have a lower technical level may appreciate the instant appraisal of such a score?