I don't think it should be expected of grandparents, especially if they have had very busy working lives up until retirement (or if they still have work commitments).
I would not have been upset if I hadn't been needed for childminding duties but I did enjoy doing it. Mind you, my husband was retired and he did quite a lot too. I am not sure I would have been too happy to do it all on my own for a whole week.
If my children were in considerable financial difficulties, whatever my preferences in the matter, I would have helped out. It is very difficult for young people these days as nursery costs are so high and quality of care isn't always great.
Our grandchildren are in their 30's and teens now so there is no need for regular child care. At the age of 75, I don't think I would be able to do a full week, but I would have done some days if it had been needed.
What I find really annoying to read is about parents who constantly lay down the law about how the child should be fed, entertained, etc, etc, and who have an ongoing expectation that their parents will take on child care. Fortunately our children have been very appreciative.