Both sides are right. I think in the end they just need to mind their own business.
If she wanted to try changing the future then go on, if he didn't want to then don't, they are not that connected with each other after all, just make sure that both of them won't have any regret for what they choose.
Agree, this is no different from the present time, in the end it's full of uncertainty. FL's future might change for the worse like she turned worse academically and only thinking about her first love... with this kind of pattern, just don't regret anything if in the end she has worse life, guy actually still die or guy survive but doesn't have any feelings for her lol and Eunjae is kinda... careless as well because he's talking to the FL, his "future" might start to change a bit as well (his future girlfriend know the FL has something to do with Eunjae).