The reason why people find it difficult to accept age mismatched couple is that the probability of the younger partners outliving the older one by several years increases, especially in cases such as this where the man is older than the woman by over thirty years. So other things remaining the same, it would be inevitable that Santo would spend a good part of her prime as a widow. Widow remarriages are not as common, especially in a society such as hers. In the case of a younger man marrying an older woman, there would not be as much problems finding another woman after her death.One can always say that it is not how long you live with your partner that counts but how you live with him. So a few good years with DM may keep Santo happy. That does not take away the fact that she may have to face life on her own for many years unless ofcourse she too dies young (which is still a possibility) or DM lives for a 100 odd years. Even if he does, Santo's burden will only increase, taking care of an ageing man (nobody can defy age associated ailments).Such things may be fine for a fiction but I am sure many of us would not want such a situation either for ourselves or for our loved ones.
Life has no guarantees, who knows maybe DM would outlive Santo. As for as taking care of a sick partner, maybe Santo could have a debilitating disease in which case DM might have to care for Santo or maybe DM would live a long and healthy life and would not need anyone to take care of him (I know people who have lived a long, healthy, self-dependant life until the day they died).
Yes, no one would want a situation like this for themselves or their loved ones, if given a choice Santo wouldn't have chosen this for herself either. But the question is when you already are in a situation like this how do you deal with it? You could either curse your fate and live an unhappy life or you could try to make the best of what life has dealt you and try to be happy. Like they say, when life deals you lemons, make lemonade.