Sheesh, Kamiya really went hard on him there. The tonal shift was a bit too sudden for my taste, it's as if we skipped a chapter. I guess the author wanted to give that brief highlight of them succeeding to bring hope, but since the manga is very much a drama, made sure to immediately bring the level all the way down again. It's about clawing up from underneath the mud with this story, after all.
One thing that slightly concerned me is that Kamiya's words carried a sense of this 'you have the potential, you're just not breaking out of the mold enough' which could backfire a little in all of this, esp with Eren's comment that there is a person who could rival her. What I mean by that is that maybe MC is far more talented than the story let's on so far, which is something I am a little concerned about. Unless of course it manages to steer it in this direction of 'he's extremely driven and focused and so just overcomes all the obstacles over time, just that now he hit a ceiling he has to smash wide open'.
Kamiya's words also made me realize that the role for MC is more of a true director one when Eren is the painter. I thought before that they were more orthogonal in their pursuits. What this implies to me is that a potential endgame is that MC could 'steer' Eren in what she creates.
The contrast the author has built up so far is that Eren is a wild beast who doesn't care about her surroundings and what people like or don't like. Meanwhile MC is the opposite - he wants everyone to be satisfied. This polarity is where they can learn from each other. So I could see him directing Eren in an advertisement job in the future as an endgame.