The key reason that Trump won the primary is because 70% of the vote was divided across 15 moderates. If the primary field was simply Trump vs. one moderate Republican candidate then Trump would have lost the primary with a mere 30% of the vote. There is a significant lesson for the Republican party in the recent primary, and I would expect to see some changes to narrow the possible number of candidates for the 2020 primary.
Up to and including Super Tuesday (at which point the primaries are effectively over), what percentage of the total vote did Trump get? In or around 30%. If he ran against one opponent, the other guy would have gotten approx. 70%.
This is a false assumption that the 'other guy' gets the other 70%. If only two choices are available, Trump surely is the 2nd pick for many of the other voters. The 'other guy' isn't necessarily so.