Council housing rent in the UK is much less that market rents. I agree that it would suit the well paid tenant not to move, and there doesn't seem to be a legal way of making them move out.
However, my point is not that it's illegal, just that it's unethical.
Why do you want a legal way to move them out, even if that breaks their social support network?
Because I don't understand why you think there is any issue with him staying.
If there isn't enough council housing, raise taxes and buy/build more housing. Someone making 70K/year should be in a higher tax bracket so it's not like he's not contributing more to society.
Would you really say it is unethical for a rich man to check out a book from the public library, because he can afford to buy his own books?
There is a finite supply of council housing. It's a zero-sum game - wins and someone else loses. I mean, we can imagine a world where we could expand the supply indefinitely but I don't think that world exists.
In general, I think benefits should largely be means tested.
You are nowhere near the limit of housing possible in the UK, and are nowhere near the zero-sum point.
Your system is set up to encourage people in council housing to make enough money to be able to buy their home. With your zero-sum argument in mind, you must think this unethical as it takes council housing away from the council.
Your system does not appear to be set up with your ethics in mind, so I hope you can understand why it doesn't seem like something others should be bound by.
However, my point is not that it's illegal, just that it's unethical.