So, why do I think Start-Up committed a plot betrayal? Let me ask you this, how did the main male character of the "story" become the second male lead of the "series"? In writing a story, you don't build a foundation that you will not use in the end because you can make it float by capitalising on a fanwar.

You might also be wondering why Han Ji-pyeong is favoured by majority of the viewers than Nam Do-san. We don't need to pit the two male characters against each other and compare their features and flaws. The story is written to make viewers experience Dal-mi's feelings vicariously. The plot's exposition clearly established who really is Dal-mi's love interest. The rising action of the story is premised on Ji-pyeong's struggle to reveal himself (and his love) to Dal-mi. They dragged this til it reaches its climax in Episode 15, making its falling action relatively short (with some lame excuses and justifications for a Do-Dal ending) and the denouement disconnected from its origin. Hence, the backlash from some of its viewers.

It really was a promising series. Majority of its viewers who watched it until the last episode hoped that the original main male character's struggle will be resolved properly even if it was virtually impossible when Episode 14 gave all the leverage to Nam Do-san. Hence, even if Nam Joo-hyuk plays Han Ji-pyeong or Kim Seon-ho plays Nam Do-san, majority of the viewers will still root for Ji-pyeong. It clearly is not about the actors but the story and how it established its characters. Nam Do-san was not even a full character until the last minute of episode 2, whereas Ji-pyeong is already a core character connecting all the other main characters, i.e. halmeoni/Mrs. Choi, Dal-mi, and the boy from the television and newspaper, Do-san.

I have to admit that Episode 16 was able to conclude other subplots well, from the apparent success of their start-up to In-jae's reunion with halmeoni and her family. Sans the plot betrayal, this series would have been one of the best KDramas of 2020.