Yes, navigation links that don't behave like normal links are also an accessibility problem[1]. That's a separate issue, though; I don't think it is a reasonable argument against supporting reflow based on perceived window size.
Mixing links and dropdowns is a 'well-known' antipattern, in the sense that lots of people don't know not to do it, so everyone is familiar with it once you bring up the subject, but most people don't think about it until someone mentions it.
This is why you see icons in the menu bar indicating that clicking here doesn't navigate you away from the page, but clicking over here instead might send you away from whatever it is you're doing here.
On mobile people may be using back button less, but on desktop it's always easily visible and accessible in multiple ways, incl. the keyboard, and at least for me it's almost a reflex to use it. Just ctrl+[ away.