Sure, but it's not that far fetched from the main story. The director got ditched exactly by the quite similar reason, so it was a valid concern. It'd be more silly, story wise, if they just brushed it just like that. Like, they should do typical SOL yonkomas if they don't want to dig more regarding the lore or characters' depth
I know this was months back but here’s my reply. Honestly this is all due to Kang’s insecurities since he is the one much older. So he thinks Yiyoung would want to experience others more before settling, so he expects a break up one day.
His insecurity was hinted in main story too. We don’t need a third rivals and jealousy plot, we need a talk that goes about his insecurity.

This whole side story thing started off bcs the director considers MC as just another boyfriend where he'll eventually Break up and go their ways.
Which kinda feels frustrating cause what was the point of the whole main story to just end up Thinking THIS.