Are you really claiming that the queen and duke of Edinburgh are just as active as they were 20-30 years ago?
I expect Anne to continue royal engagements as long as she can - and surely when her brother is on the throne. However, slowing down by the time she reaches 90 (if she is still alive and able to carry out royal duties) does seem far more likely than keeping up with her current speed.
Phil has retired and I think that before he gave up he was really pretty fragile and not up to any work... and the queen still does a lot for her age but she is in her 90s now and she cant keep up work indefinteily. Charles is taking over more of her work