It is an inevitability. Just like with the Lib Dems/Labor in the UK. Plurality first past the post systems always lead to two parties due to Duverger's law (http://en.wikipedia.org/wiki/Duverger%27s_law). If the right gets behind one party and the left splits the vote among two parties, the right will win again and again. Just look at Canada's 2011 election results. You think the conservatives aren't going to win again in Canada in 2015/2016 without a Liberal/NDP merger?
Yes, Duverger's law predicts a two-party system. It's not
quite directly applicable in Canada for a few reasons, chief among them being the decentralized political structure, but that's not terribly important so we'll ignore that. While there will be a party of the right and of the left in Canada in the future, it is infinitely more likely that the NDP simply supplants the Liberals. Already places that have always voted Liberal are voting NDP. Others are voting Tory. That's how it works.
The idea of a Liberal-Labour merger in Britain is patently ridiculous.