You asked, "When does it stop being a problem?" Perhaps what you meant by "it" is a touch vague.
If you included a bunch of racy art in the 2024 PHB, it would be a serious problem for that book. So, in that sense, it hasn't stopped being a serious problem. As soon as it is present, it'd be a problem.
Is the current level of sexism in the art of the current WotC D&D core books a problem? Probably not, insofar as I don't think we see much complaint about the current art. If they kept similar art direction, I don't expect it would be an issue.
Actually, I do think there definitely is a problem with the art in the current edition of D&D. In my opinion, I think the majority of these images define beauty and heroism in humans and human-like individuals as being slim, youthful, muscular, and having highly regular features, a stereotypical and unrealistic look that is wholly devoid of any substance and realism - stylized, empty, and dare I say Manga-esque - and which I would consider problematic especially re those of the young internet generation that do not look like any of that. I do believe that there has been some recent research into how images like this have a negative, even dangerous, influence on children and teenagers growing up with them.
5E PHB, pp. 25, 29, 38, 42, 44, 51, 55, 61, 64, 67, 70, 76, 90, 94, 99, 105, 125, 129, 130, 134, 137, 138, 140, 148, 167, 192, 199, 200, 206, 217, 220, 232, 253, 262.
I do believe that is the vast majority of the art in the PHB.
Now, how and whether this relates to the subject of "cheesecake" or "sexual content" is, I think, matter of opinion but I suppose it could be argued that "cheesecake" art back in the day served to express notions of heroism and sexuality the authors believed "appropriate", just like the creators of the art of today may believe theirs to be.
And who knows how folks of the future will define the art of today?
In my opinion.