Why is this such a dumb counterfactual? Let me count the ways:
- Unlike parliamentary elections, individual voters could not vote for the full range of candidates--they voted in either the Democratic process or the Republican process
- Unlike parliamentary elections, the primary/caucus processes in the US were not simultaneous but sequential. Which meant that folks voting later had few choices and were reacting to the outcomes of the previous primaries/caucuses
- Turnout? Primaries have lower turnout than general elections. In primaries, the most passionate tend to turn out, so you tend to get more extreme wings of the parties show up. Indeed, this is one reason why Trump is so confused and ill-prepared--he thinks the electorate is the same for the general as for the primaries, so he just has to do more of the same.
- Um, this conflates type of political system with electoral laws. There are parliamentary systems with majoritarian electoral laws (that would be most of the Westminster places--NZ is wacky). And presidential systems can have a variety of rules for picking the president and then different rules for the legislature.
It would probably be far more accurate to take a look at surveys of the entire electorate on a variety of issues and then figure out how they would shake out if we had a proportional representation electoral system combined with a parliamentary system.
Sorry, to be a killjoy, Economist, but this is the kind of infographic that creates ignorance, not enlightenment.