I think if they're both earning reasonably well they could perhaps tighten their belts a bit and bear the cost of a nursery for a few years. If your daughter truly does not want her daughter to be looked after by strangers then surely she (and possibly her husband) can explore going part-time with their employers, or she can go part time and let the other granny do some childcare?
In the situation you describe, where you have had years of caring and have been looking forward to some time to yourself, I think it's perfectly reasonable to say you would prefer not to do it. You don't say whether you have a partner who would also be helping. If not, I think it's very tiring to look after a baby/young child single handed, and requires a lot of energy.
I enjoy having our grandson two days a week, but my husband is also at home and we share what needs to be done. My husband might take him out for a short walk or keep him occupied while I get the lunch ready, etc., and vice versa. When you're on your own, it's far more restricting and tiring.
I would feel slightly differently if my daughter and son-in-law were desperately hard up and not earning very much. Personally speaking, I would find it difficult to refuse to offer some help in such circumstances but, from what you say, that isn't the case.