Very nicely put and what I would have mostly expressed about this particular comparison. Putting aside size of the zoom for the moment, any given zoom is optimized for certain focal lengths in it's range, often at the expense of some others. This is especially true to mid range zooms. So often one finds a given focal length in a zoom that "betters" a single focal length lens but the zoom can be much worse at a different focal length in it's zoom range vs. another single focal length lens.Don't worry, I'm not upset: but these are both lenses I have reviewed in depth and I simply don't think it's a simple as zoom versus prime. Many, many people for example think that the Nikkor 14-24 is the best ultra wide there is for that mount, including primes. Back to Sony: the 35 f2.8 is smaller, lighter and more versatile and a good copy is not only 'good enough' but is often much much better than that. I prefer it, for example, to the Sigma 35mm ART lens because it has much more predictable shape of field of focus. The 24-70 is huge and heavy and more expensive and isn't as much of a zoom as you might expect because of its weakness at the wide end. Sure it's great at 35mm but then so is the Sony Zeiss 24-70 F4.
Horses for courses but apples to apples....
Comparisons are not always easy to make. Even when comparing one single focal length lens vs. another, size and weight and design objectives come in to play which relate to ultimate performance. Just look at the Otus. Part of extracting great performance from it I suspect required the lens to be that large, so simply comparing it to other 50-55mm f1.4 lenses, does require other considerations.
Dave (D&A)