I’d say the problem is more in the two party system. If you think about all the opinions one can have about how a country should be run, it’s plain weird to only have two choices.
Even if you consider two-party system as failed multiparty one, it has advantage of being already past that bifurcation point. And, considering that alternative route is death of parliamentarianism, it's a nice advantage.
Yes, same failure can happen in two-party system, but chances are different - 'usurper' party has to fight against single opponent, rather than against multiple smaller ones. Question of 'whenever opposition can unite' is trivially true if opposition consists of single party.