well, this just shows which target market nikon is aiming for 
i am pretty sure a plastic mount is cheaper but by how much? if its just $30 id gladly pay that extra $30 
But a metal mount would not make the rest of the build or optics any different.
I have seen plenty of Nikon lenses that are poorly constructed (i.e. scarily wobbly barrel in AF D zooms of 1990s, look at the first 24-120 or even the 28-105) but have metal mounts. Newer releases in the consumer category have become less wobbly over time, in fact the 16-50 DX Z feels really nice in terms of mechanical feel compared to kit lenses of old in that same price class. Even primes such as 50/1.4 AF D had somewhat wobbly barrels and even to such extent that my copy (purchased new) would "jump" visibly when autofocusing in vertical orientation, and sound a bit like something is wrong and it was made worse if a filter was used. But there was nothing wrong with the lens as such, it was made that way. The 80-400 AF-S has a metal mount and its autofocus would stutter a lot and the lens would vibrate easily due to shutter shake or wind. Poorly made but with metal mount.
As for making something in between the compact primes and the S-line; my interpretation of the situation is that the manufacturer is trying to make the lenses distinct enough to expand the size of the overall market for Z lenses and satisfy a larger customer base. If they are too close, the less expensive one might take the majority of the more expensive lens's sales. While it may be appealing for some of us to have a lens that is a perfect compromise between size and quality, some customers really want as light as possible and some want the highest quality possible. Those latter two have marketing appeal (i.e. the best lens in a category or the smallest and lightest), the perfect compromise might not sound so amazing (even if it is really good and practical). It might not be possible for a manufacturer without market leadership position to supply to three levels of moderately fast aperture primes at each focal length (does that translate also to 3 levels of zooms as well? They kind of have that already if we count DX). A lot of people are asking why Nikon are focused on making numerous lenses in a narrow range of focal lengths and leaving out other ranges.
I do want well-made lenses for my own use but I'm more concerned about collapsing / telescoping designs than the plastic mount in the compact primes. Of course, if the lenses turn out poorly built then that's not a good thing, but I'd give them the benefit of doubt and see what they are able to make and how the lenses will perform.
