I thought you meant with their current workload.
I don't think J&M opting out completely is a realistic scenario. Not unless they decided to move to France or something like that. And that would be most odd IMO.
Yes, Benedikte did her bit but not to the extent that she is doing now, she after all had children, husband and the estate at Berleburg to keep her busy.
Queen Ingrid also stepped in on a relatively low key basis. And as you may recall she also stepped in as Rigsforstander, even is she was not in the Line of Succession.
So in a scenario where J&M opted out more or less completely, it would not only be possible that Mary would from time to time be Rigsforstander, it would be inevitable IMO.
(Which would make Mary the de facto head of state from time to time. There is a particular website where they would have a collective heart attack, should that happen... )
Sorry, badly put: I meant with their current workload, but if they continue with that if Frederik becomes King next week, it would, for me, be kind of "opting out".
Several members of this board would be having the collective heart attack too...