A sad ending without them being together would really make me sad for Woo Joo.  She just seems so broken to me and Dong-jin's unmitigated consideration and kindness would go a long way in helping her to heal and smile more.   Dong-jin too would benefit from knowing he would be loved by Woo Joo and become part of a healthier family dynamic.  I so want this for both of them.

Admittedly, I'm never a fan of open or ambiguous endings, but I particularly feel these two need each other to continue healing.  It would be beyond difficult to find others who could help them move on to the best and happiest version of themselves.

Alas, like so many others, I'm far too invested in these two and their happily ever after.