It really all hinges on how successful the Tories are over the next 5 years, to be honest. If things actually improve in our society due to Tory policies, or if things stay pretty much the same / only get a bit worse and the Tories manage to spin-spin-spin-spin it into positives like they somehow did this year, then yes he is unelectable.
If Tory cuts and austerity policies are proven to be an absolute disaster and they don't manage to successfully cover up the damage they've done, then no, he is not unelectable. I honestly feel that if, after 10 years in government, the Tories haven't proven the worth of their economic policy to the extent that the country / towns / lives start to improve, then people will be ready for a change.
Of course, like I said, the probable outcome is that nothing at all will get any better but the Tory press will once again employ a 6 month campaign of smoke, mirrors and figure-fiddling to make it look like things are super awesome
.