[canistr]

That doesn't mean that the two largest parties remain the most powerful forever. That's what makes it a theory.

For instance, look at state of the Canadian Parliament. For the longest time, the Conservative and Liberal parties were the two largest parties generally swapping power every couple of years. Now we see the Liberals have severely dropped in the power with the rise of the NDP to become the Official Opposition and being the second most voted party in Parliament. Even the Conservatives, who the hold, only garnered 40% of Canadian votes.

Again, what you're saying is a theory and it isn't even always the case.