i do understand that it. isnt 100% what you are after, but i do think the metric can work for that purpose in many cases

I had same issues when i wanted to create stickiness metric. please let me know if this solves it.I created a custom metric. Created segment on 30 full days, added people(visitor) metric, changed the people metric to grand total instead of standard.  (People is the same as visitor, just CJA name)  ...