I personally like that QMII feel the way she does about abdication, it shows her strong sense of duty to her role as a monarch. Also, it isn't certain that Frederik will live as long as his mother and grandmother, something unexpected could come up. Take for instance King Frederik IX, QMII's father, he died unexpectedly from the flu. Say QMII abdicates in two years and a half when she's 75, at that time Frederik will be around 47 and little Christian will be around 9-10. Then say thirteen years go on, Frederik will be 60 and Christian will be about 23, however something unexpected and tragic happens and Frederik dies, then a 23 year old, unexperienced Christian will be King and that wont be nice for anyone, least of all for Christian. At that point, QMII would be around 88 and possibly still alive and fit to be a monarch, and her not abdicating could have spared Christian from a lot of trouble. Obviously this is all hypothetical and I'm sure Frederik will live for a very long time, but that's just one of the reasons why I think abdication is a bad thing; it could go well but it could also go very wrong.
On the other hand I definitely think that it is fair for an old lady (or man) who has reached the age of retirement with several years, to have some years for the family without having reigning a country as her/his main priority. I think it depends on the situation.