Very good, you are starting to understand my point.
Yes, a 16-18mm f4 zoom is less of a compromise than a 14-30mm f4. But why does it matter if a 14-30mm f4 can be made to be excellent as the Nikon Z is?
Similarly, it’s obvious that a non collapsing design will be more robust and superior optically because the need to make it collapsing has a tremendous impact of where the elements can be located etc... but does it really matter if a collapsible design can be made to be robust enough and optically good enough?
Cheers,
Bernard
There's no difference in optical quality between internal focusing and extending designs. If anything, extending designs require fewer optical compromises, since you aren't restricted to a fixed front element and a rear element that can only move a few centimetres.
The issue with a 14-30mm lens (same with 24-105, 50-500, 12-24 and other similar large zoom ranges, telephotos being much more forgiving than WAs/UWAs) is not that they cannot be made sharp, but how they fit into a role.
F/4 zooms really fill two-and-a-half different roles. One is to be a cheaper alternative to f/2.8 zooms, for the budget minded. Another is to replace a bag of primes in a portable high-end kit, for those who need sharpness but not wide aperture, rather like Leica's Summicron line in relation to its Summilux line. The half, which overlaps both other roles, is to be small and lightweight - in absolute terms when considering the budget/consumer category, and relative to the bag of lenses they would replace in the high-end role.
Lenses like 12-24/4, 14-30/4, 16-35/4 and 24-105/4 fill the first role very well, when the aim is to cover a wide range of focal lengths with minimal weight, size and cost. Sharp as some of these lenses may be, though, they aren't as sharp as primes. This makes them less than ideal in the prime replacement role, particularly in front of high-resolution 42/45/50/61MP sensors.
Generally, a prime will be sharper than a zoom at the same aperture. The thing is, a typical prime will have an aperture of f/1.2-1.8, while the zooms we are dealing with here are f/4. The zoom may require some compromises to achieve a variable focal length, but the prime will, equally, require other compromises to achieve a wide aperture. Make the zoom range narrow enough and you can make it about as sharp as a prime, within its aperture limits (or, at least, from f/5.6 onwards). This makes such lenses ideal for landscape, archictectural (with distortion correction) and general travel photography, where you need something as sharp as possible, but not necessarily wide aperture (and you can easily add an 85/1.8 or similar if you want to shoot portraits as well, or a lightweight 70-200/2.8 for action, since the optical penalties for a zoom design seem much less pronounced at telephoto focal lengths). Extend the zoom range too much, though, and you'll never achieve this sharpness, making the lens less than ideal as a prime replacement.
No, this setup won't be as light as a kit with f/4 zooms with a large zoom ratio. But it's not meant to be, and doesn't replace those zooms, despite the similar-looking specs.
So, you might go 14-30/4, 24-105/4 and 70-200/4 if you want an ultralight, decently sharp kit.
Or you would go for a 14-21/4, 21-35/4, 35-105/4 and 100-300/4 if you wanted an ultrasharp travel/landscape kit that still weighed less than the bag of primes it replaces. Add in a 35/1.x, 85/1.x and a macro and you'd have an all-purpose travel photography kit that weighs about the same as an f/2.8 zoom event photography setup and is much more fit for purpose.
So, two types of f/4 lenses. 'Sharp enough', with a wide zoom range, for convenience and ultimate portability, where the aim is to minimise weight for a given capability. And super-sharp, with a restricted zoom range to achieve this, as part of a prime replacement setup, where the aim is to maximise capability for a given weight.