Marathoned this in a day or two, I get some of the criticisms like the frustrating, sluggish development, him blushing all the time etc but dunno why people think this here is bad or depressing.
By now we know how she is: she's tough, hardened, she doesn't open up to anyone and is perpetually on guard around others, even close ones.
The moments where she did open up to him, like this one, or when she asked him to make her movie... those moments are rare, genuine, and special for someone like her. She hasn't said this to anyone else, he's one of the two people, or maybe the only one she's ever talked to about this.
Knowing that, when someone like her lowers her guard and lets you in, leaving herself vulnerable, it means a lot. There are more ways of having an emotional moment, a deep bond, a real appreciation, other than saying "I love you".
This isn't as much about Kazuya's growth as it is about Chizuru's. He cries because he knows all of this instinctively, and finally felt useful to her, like he finally gave back to her for real.
It's a beautiful moment and you completely missed it, and even if I happen to be wrong, I'm still glad I got something more positive out of it than a "lol simp."