I think you are ignoring the effect that aging has.
You say they are in their 70's and are very independent. That's fine - my parents were exactly the same - they were even running a business. However, they are now in their 80's many of their friends are going or gone, they attend funerals just about every week, they are in poor health and life is looking grim. Dad is the last of his generation, and mum looks at her aunts, and some of their cousins, who are in (or lived into) their 100's and hopes she dies before she reaches that age. Remember your parents-in-law are not very far from average life expectancy.
You say that there is only one of their parents they have not outlived. To me, that is a warning sign that things are going to start to go wrong sooner rather than later. Remember that most people become much more dependent in the last 8 years of life - and that is sooner rather than later. Make sure that wherever they move is suitable for aging in place - has wide, wheelchair friendly passages, places for wheelchair turning circles, has no stairs... Make sure facilities are available (appropriate medical specialists, care in place, meals on wheels...) - small communities in particular often don't have these facilities, and they are necessary if your in-laws are not to move again.
My parent's live 7 1/2 hours drive away. They wanted to move here when they were in their seventies, but I wouldn't have been available all the time, and they were (and still are) part of the community. Each time I go shopping with one of them, they meet at least one person in passing who stops to talk to them. I'm sure they would have had a difficult time adjusting to a new community here. How easily do your parents-in-law mix and incorporate new friends? Each time I have seen an elderly person move from one community to another, they have become more frail, more dependent and less cognitively aware. Social change is very hard for them.
In the last 10 years both parents have become very hard of hearing. Both have developed eye problems. Dad only has sight in one eye, and marginal sight in the other and today is having a cataract removed in that eye, because otherwise he won't be able to continue driving (I am extremely worried about this, as the other eye became much worse when they removed the cataract in it a few years ago). They both have major problems moving around. Remember that they were running a successful business 10 years ago, and you will see what the next 10 years probably will mean to your parents-in-law.