This Ending is a story about exes realizing their mistakes as they look at each other's younger selves, remembering the past all while being scared of when things return to normal, back to the reality when they broke up. Even in the end, there was still the lingering worry of whether they'll stay together or if there will come a day when they'll say their goodbyes, and I find that it was that part that left most people unsatisfied.
But it's the truth though? It's the realistic part where them as characters don't know their own future, much like how we don't know our own.
The important thing to note there is that Suhyeok said it doesn't matter. Those last few lines is not him doubting the new chance for their relationship, it's saying that those uncertainties definitely exist but they don't matter anymore. Because what matters in this story after all the time travel shenanigans is the present, the moment right at the end when they're facing each other again. They wouldn't know if they'll change again or make the same mistakes, but it's enough that at that moment, they learned from it and are trying again. They love each other, it's a fact that will remain true in that present no matter what the future holds. And I just think that's the best way this excellent narrative could have ended.
P.S. Also remember how Suhyeok is the type who doesn't want to get his fortune told because he'll end up worrying about it? It's just easier and better for them to not know the future, and where that leave us is to each their own, for me I think it's a happy future