To put it simply:
He wouldn't use this promise to hold Wu Wenxin emotionally hostage.
The her from the past truly liked him.
If the her from now still liked him, then this 'reward' could be continued.
Wu Wenxin: Should we get to know each other again?
Shi Yi: That's what I was thinking.
Wu Wenxin: Huh, why?
Shi Yi: The you from before is the you from before, I don't doubt the sincerity. But your attitude now is more important, it should follow your current opinions.
Wu Wenxin: Oh, so that's what you think.
Shi Yi: Yeah, what about you?
Wu Wenxin: I just feel I've changed quite a bit since I was a child, don't you find that weird?
Wu Wenxin: But...
Wu Wenxin: I should be a little better looking than when I was a child [cold sweat]
"..."
Shi Yi had always thought he lacked emotions, not understanding what it felt like to have a crush, even when girls confessed to him, his heart would not waver.
But now he discovered, it was because the 'person' was wrong.
