Once again, NDP has won Opposition before (which to those unfamiliar, means that it held more legislative power than all other parties aside from the one that won the election). It is a viable 3rd option and likely would have won the national election roughly a decade ago if Jack Layton hadnt died of cancer during his run.
And because of how our legislature works, if the winning party wins by a minority (Less than 50% of legislative seats), the NDP can team up with the opposition to outpower the party that won and use that to attempt to get legislation that it wants pushed through as a compromise.
TL:DR your last paragraph is false and only serves to entrench the 2 most powerful parties. Canadian 3rd parties are VASTLY more viable than American ones