Over 70s who are still working will be concentrated mainly in the early 70s, and regardless of age would mainly be the ones who would pass a driving test fitness check just fine anyway. My dad is late seventies and still does some consultancy work part time, and he drives extremely well so a requirement to do a check would not be an issue.
If someone is in their 70s, is unable to pass a driving fitness check yet still somehow employed, which will be only a few people, then I am sorry but they will need to suck it up and accept that they will have to increase their travel costs somewhat by switching to cabs whenever public transport does not work, and giving up their car. It is not OK to endanger people on the road, it is as simple as that.