I don't agree that this kind of time skip is as common as you're making it sound. Even in stories with 20- or 30-year separations, there's usually a significant reason behind them, whether it's the historical setting, the lack of communication methods, or some major circumstance keeping the characters apart. Here, that justification isn't nearly as convincing. We're talking about an era with mobile phones, not ancient history where contacting someone was practically impossible. They could have communicated, looked for each other, or at least tried to find out what was happening in each other's lives. That's why this ten-year gap feels so unnecessary to me. The problem isn't that long separations can never work; it's that they need to be justified and written properly.
Tbh I agree with you that the time skip wasn’t necessary. I didn’t like the choice myself. But, regarding whether the justification is convincing or not is up to the individual reader I guess. Personally, I think it’s convincing. Dohwa’s, not so much imo. Also I don’t think Eunhyuk’s trauma/depression justifies it, but it explains why. He has to make up for it now and he’s making an effort to do so now even if it’s frustrating to read.

Eunhyuk’s supporters are something else. You won’t even consider someone who ghosts you for a year as a potential partner, but apparently, someone who ghosts you for TEN WHOLE YEARS is still a valid option? Be so fucking serious. Ten years is not a small amount of time. People change drastically within one or two years, so imagine an entire decade. People get married, have children, change careers, go through life-changing experiences, and some people even die within that time. An entire decade can completely change who a person is and what they want from life. So how exactly are we supposed to believe that someone can just disappear from your life for ten years and still be considered a romantic option as if nothing happened? And if you think ten years is an insignificant amount of time, you seriously need to rethink your understanding of time. I genuinely don't understand why the author chose a TEN-YEAR time skip, because that's not some brief separation where the characters can realistically pick up where they left off. That's an entire decade of living separate lives. The author could have chosen a much shorter time skip, but ten years? That's an entirely different lifetime for some people.