First off, there is nothing wrong with the old age, if the person is fully capable of performing their professional duties and has been helpful to you so far.

Secondly, even if your fears of her retiring or dying happen to materialize, it doesn't make sense to end the good experience before it ends naturally. It's like being afraid to invest emotionally or in any other way in any good life experience because it might and, most likely, will end. Everything ends in this life, every single life experience. If we can't enjoy it fully or make the full use of it in the present just because it's going to end some day, then we are not living.

Yes, there is a chance that she may retire, but, ethically, she is supposed to give you a very advanced notice about it and to prepare you for this event gradually over a long enough period of time for you to get used to the idea and to be able to transition to a new therapist. So, it's not going to happen out of the blue, again, assuming that she fulfills her ethical responsibility.

As far as her "croaking" on you, I think, early 70s is too early for that to happen if the person is relatively healthy with no life threatening illnesses and if they take a good care of themselves. Hopefully, this is the case and her health is as good as it can be at her age. But, if it does happen, I believe, it's better to deal with that at some point than to interrupt the generally good therapy process that has been helping you so far and to embark onto a long and exhausting therapist hunt, which would take God knows how long until you find someone that would work for you, which may not happen. Just think about all that time you'd waste on looking for a new therapist when you could continue to use it for your benefit if you stay with the therapist you have now.

What you can do though and what I'd highly recommend that you do is to talk to your therapist about your fears and ask her how she plans to take care of her clients in the event of her retirement or sudden death. She should be willing to explain to you how she is prepared to deal with this. Therapists who are close to the retirement age are ethically obligated to have plans and make arrangements in case of such events. Of course, oftentimes, the death cannot be foreseen or anticipated, and, if that happens, it is what it is. But there are cases of long time chronic or terminal illnesses that, in combination with the old age, can be anticipated in advance. In those cases, the therapist should make their best effort to make arrangements that takes care of the clients.