hi, it isn't a stupid fear ... it's a very valid one and one it would be good if you can address. even if ageing is a challenge to her, it is still something that is important for her to discuss with you especially in relation to the questions can'texplain suggested. therapists, as with everyone else, die ... and it would help you to know what would happen for you as far as finding out; who will give you support and help you through the grieving. My therapist did die in her mid-70's and I was completely unprepared for it at the time, so instead of labeling your fear irrational try and address it if you can