From what others have said, it is a lifetime tenancy. So if their income is assessed annually, their rent will have increased.
It is irrelevant if they can afford to buy a house now. It matters if they could afford to buy a house at the time of the initial assessment. People living in council houses doing well for themselves isn't something new. It has always been that way going back to when I was growing up. People weren't kicked out of their homes then and they won't be kicked out now.
In fact, you can say that it is good for the country as a whole. I would much prefer to see someone getting a council house doing well for themselves and their family instead of giving it a generations of the same family who contribute nothing to the country but just take benefits. At least the people doing well for themselves are paying taxes and the chances of future generations living on benefits is reduced.