First off - nobody uses Bootstrap anymore, and frontend is way more than just CSS. Just from the comment I can infer that you probably know very little about frontend, yet have the confidence to disparage frontend as a "solved problem". Your comment is a perfect example of the lack of respect towards frontend.
Let me ask you, what in software engineering isn't already a solved problem?
I'm glad you used that wording, because it seems that the most optimal way to work in this industry is to work on those problems that aren't considered already solved. I think that very very few of the problems that engineers work on haven't already been solved. But perhaps that's one lens to look at things in order to decide what kind of work has the highest ROI in working on. Clearly from a tech career perspective it's not frontend.