Surely there are considerations other than maximum sharpness, such as price, weight and general usefulness.
To buy a lens simply because it is the sharpest currently available at a particular focal length, without regard to cost, weight, flexibility and ease of use etc, would seem to me to be a bit short-sighted if one is an amateur photographer.
If one is a professional earning one's living from photography, then that's a different matter. The equipment one buys can be depreciated over time as a tax write-off. The professional will tend to have a clearer idea of the type of lens he will need for a particular job, and if the cost of hiring a pretty model for just one day, for example, could be greater than the cost of the best 85mm prime lens available, why not buy the 85/F1.2 even if one has only a few assignments during the next few years when that particular lens will be really useful.
Whilst it's true that camera bodies in the digital age are updated and upgraded more frequently than are specific lenses, and whilst it's true that each increase in sensor resolution will have a greater impact on the sharpness of the resulting image when an excellent lens is used, as opposed to merely a good lens, the facts are that most lenses are eventually upgraded, even if the upgrade is no more than the inclusion of image stabilisation, and/or a macro capability. I'm thinking here of the Canon 70-200/F2.8 L USM and the 70-200/F4 L USM which always had a reputation for being the finest zooms available, but without image stabilization.
Supposing one had bought either of these lenses in 2005 to use with one's first full-frame DSLR, the Canon 1Ds2. Two years later Canon introduces the 70-200/F4 with IS, and the lens is at least the equal of the non-IS version, in terms of sharpness, and apparently slightly better.
Yet another 2 years later, Canon introduces the 70-200/F2.8 with second-generation IS. This lens is also the equal of the first non-IS version in terms of sharpness, and clearly better than the first IS version which appeared in 2002.
Of course, if you always use a tripod, then the lack of IS is not an issue, but occasionally a company will introduce a completely new lens in terms of focal-length range and maximum aperture, such as the Nikkor 14-24/F2.8 with a constant maximum aperture throughout the range.
I know that I would rather have a high quality wide-angle zoom than two or three wide-angle primes that not only cost much more in total than the single zoom, but weigh in total significantly more. If the image quality advantage of the primes is very marginal, as appears to be the case when Zeiss or Nikkor primes at 18mm, 21mm and 24mm are compared with the 14-24 zoom, then the choice is a no-brainer.
