it's honestly impressive how much mileage they get from that boobs gag, it somehow doesn't feel old yet
I'm not sure it's really a gag, it's more the mechanism through which the whole manga is conducted. Which is clever, except that boobs are such a non clever topic, it removes the highbrow-ness of that concept. Which is pretty clever in itself, and also funny. Anyway, the reason it's not old is that there's so much self-awareness and irony in it that it helps, and also that if it was gonna be old, it'd be old on the first page.
tl;dr it's not old because it's a narrative device, because it's self-aware, and also because the only people still reading would be people who aren't put off by it.