Similar position. Both parents and in-laws have suddenly, i.e. within a year or two, gone from busy and active people enjoying their retirement, voluntary and paid work, holidays etc, to elderly people who are struggling to cope. They are all in their mid seventies. Both sets of parents live a couple of hundred miles away from us - in different directions, so we don't see them as frequently as we might like and the changes between visits is striking.
In both cases sudden and serious illness has played a part. My DF went from fit and healthy to dying of heart failure within months, following a cancer diagnosis. My MiL has a serious and untreatable condition which has left her unable to walk unaided, and even then only for very short distances. FiL is struggling to cope, both with the situation and with the extra caring and household responsibilities. But in addition there are other things like no longer wanting to drive after dark (which is understandable) and no longer wanting to use email or the internet (which is a little frustrating). Just 2 years ago he was organising festivals, so this is a big change. They are just about coping at the moment, though it won't take a lot to tip the balance and they are already relying a lot on the kindness of neighbours (we have each others phone numbers).
Short term DH or I can go over to help, but long term, we can't live there and they can't live at ours (no room and we have stairs). Downsizing or moving anywhere (e.g. supported housing which isn't miles from the nearest shops, drs etc) isn't something they could cope with at the moment.
The last 6 months have had us going back and forth between our various sets of parents. That has been all my Ds's school holidays since Easter. During term time, we have to take turns so someone is at home with DS. Our work is flexible in that we can take time off, but that does mean we don't get any income, and that isn't a long term solution.
We did have some plans in place, LPAs etc, but given the speed at which things have happened, and the emotional toll on everyone concerned, those have not been enough. What might be a practical solution in an abstract situation is not what any of the people involved actually want.
Sorry no solutions, just sympathy. It is hard.