I would call it tempered (forced to compromise on it's nicer features, like representation, to save core ones, like turnover of power) rather than failed (when

I would call it tempered (forced to compromise on it's nicer features, like representation, to save core ones, like turnover of power) rather than failed (when even core features are no longer present). 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.