40 miles is nothing for some but a difficult journey for others. My children have asked me not to move too far away when I get older, at the moment one lives about 5 miles away and works nearby, the other lives about an hour and a half motorway drive away. I've lived here for many years but still have a hankering to move back home, but, once Mum is gone I have no family there, and to visit either of them would mean a long journey, so thought that the best thing to do would be to stay put, I visit Mum [in normal times] for 5 or 6 days at a time or maybe a week, am lucky in that I do work from home. Neither child could do this with their job. When Mum is gone I can still visit the area, as a tourist. My inlaws lived a couple of hours on a good day journey in the car on the train, no buses in their area. My brother in law lived not far from us here, about 15 miles away. To help them meant either all weekend, or taking leave for him and his brother, and when a crisis happened, as crises do happen it was an absolute nightmare, they were reluctant to get in carers so it was a worry....life would have been so much better for them had they moved somewhere within easy reach of both their children, there is a town that many retired people live in about 15 minutes drive from BIL and us, someone could have seen them every day, most crises would not have happened. They would have seen all their family often and honestly life would have been easier for everyone. But even though they rarely went out, and wanted to stay for their friends, to be honest, those of their friends who were still alive couldn't visit and they turned down offers of help from the statutory authorities and friends on the grounds that their children were looking after them, they did not seem to realise how hard it made it for those children, who still needed to work. When MIL died and FIL HAD to be moved nearer he had so many more visitors and much happier life. It wasn't just one difficult year it was several, with crisis after crisis, and the strain on the distant children was horrendous. I had a difficult year when my dad was unwell but was able to visit and stay so while not ideal it was not nearly as bad as with the inlaws.
If you would make it difficult for your children like this, do seriously consider whether it would be best for everyone if you did move. It is obviously harder to move as you get older and frailer but it might be better even if it is later than ideal. And get yourself powers of attorney, if one of you develops dementia it is too late to do and going to the court of protection is a dearer way to do it and a harder way.
Everyone's situation is different, so no one can say do this do that to anyone else, but please consider what will happen, have crisis plans and do your best to protect yourself when you need help at whatever time in the future. 40 miles is not too far away in a one off crisis but if the situation becomes chronic it is a big strain on the person needing to do the visiting, it's not 40 miles is it it's 80 miles there and back. I have friends 40 miles away and usually when I see them it's an overnight stay. If the OP's son has a business it is perhaps unlikely that he will up sticks and go off elsewhere, but to be honest if at any point he does and you are not nearby there will not be much to keep him there.