Keep in mind that with the elimination of the Liberal Party, there'd be no coalition for Harper to rail against.

My point is that there would be a vacuum in the centre of the political spectrum. Since voters fall on a bell curve, most of them occupy this central position. In order for Harper and his Conservatives to get enough of them to vote for him more than once, he will have to shift to the centre. Same goes for the NDP. Looking at the two parties, it is likely that the NDP have more pragmatists in it's ranks than the former Reform party, making it easier for them to shift to occupy the vacuum left by the Liberals.

Ether event would be unacceptable to the hard right that makes up Harper's base. A shift to the left for the Tories would, in the eyes of this base, make the Tories no much better than the Liberals. Conversely a shift to the right by the NDP would also be unacceptable to Harper and his base as it would be a viable contender for government, preventing the Tory wet dream of Harper government forever, just like in Alberta.

Speaking of Alberta, even though there has been a marginal shift to the right (more to the centre from the centre-left) there's no great call, even in Alberta for a hard turn to the right. Not even the Wildrose Alliance has dared to openly campaign for the elimination of public health care or the repeal of the Canada Health Act. Why? Even the leadership of the WA know that would be a vote killer. It is unclear if Harper knows that, but that remains to be seen.

So in short I agree with you only in the short term. In a longer view the destruction of the Liberals would either shift the Harpercons to the left or pull the NDP right to soak up the majority of votes that lie in the centre.