While focussing entirely on the problems of the Lib-Dem coalition partners the article conveniently ignores the equally dire state of the Labour Party. The latter has a lacklustre (some would say 'lame duck') leader, was slaughtered in Scotland, is doing badly in much of England and did not do as well as expected in Wales. Milliband has also failed to heal the rift between former Labour traditionalists and the quasi-Tory election opportunists who now dominate the party.
However, I think Cameron has made it perfectly clear he intends to continue for the full five years unless forced to do otherwise. I'm sure he would welcome an early general election but does not want to be seen to be the person who triggers it.