Look at the unrest in Hong Kong over China pre-approving candidates for political office. In a place that had democracy and is transitioning to Chinese rule it is obvious that very many people don't think they are getting "more choice" with their locked in list of candidates. I am not championing the US system (it has many flaws) but at least Democrats and Republicans have vastly different ideologies that people can choose from. That citizens from both parties tend to pick moderate candidates (that are closer in ideology) does not lessen the real choice the people are given when voting in elections.