As with so much of welfarism, council housing incentivises all the wrong types of behaviour, and Cameron's tinkering will change nothing. If the idea is that those tenants who have jobs to support themselves are to be turfed out of their below-market-rate rents, you introduce a new reason to remain resolutely at the bottom of society, or at least appear to be so.
It is not hard to imagine a couple deciding to fake separation after five years of occupancy, in order to reduce the stated level of household income, nor for someone to turn down a job if in doing so, he'll lose his home. So the rule would add to the downward pressure exerted on anyone caught in the honey-trap of welfare.
If the full abolition isn't likely, here's my suggested tinker: After five years, you have to buy it or move on.