Means testing of benefits is usually complicated and expensive to administer.
Simply leave it as it is and then tax it back off higher earners.
The problem with having diminishing levels of benefits depending on what you earn is that it can discourage people from trying to better their situation.
Take an average family, two parents two children.
Now with the current tax situation it doesn't really matter all that much what they earn between approx £15,000 and £30,000 because of the way the child tax credit system works.
I forget the exact sum, but at some point above £15,000 you start losing tax credits at a rate of 41p in the pound.
So for every pound earned above approx £15,000 they lose 41p in tax credits, 12p national insurance, 20p income tax, an effective tax rate of 73p in the pound.
If in a couple of years time we are talking about the same family but one has recently graduated and has a large student loan then once he/she gets above £21,000 he/she will lose a further 9p in the pound making an effective tax rate of 82p in the pound.
If any extra income they may accrue as a family involves any extra travelling or childcare costs then there is no net gain at all.
It could well be argued by those without children that this family was lucky to get tax credits in the first place, but it is also clear that whilst they do the system completely discourages them from trying to earn more money and better their situation.