But don't forget that the FF and APS-C lenses from the analogue era, and also many from the first 10-12 years of DSLR photography, cannot transmit the resolution of the current digital sensors.
At 15-16 MP you can already see significant limitations in resolution.
I myself used to collect high-quality lenses from Zeiss, Leica etc. and used them with adapters on digital cameras, but after a resolution test about 6-7 years ago I sold them all.
The old lenses on my Pentax 645z do pretty well, particularly with good technique. That's an example of adding pixels by making the sensor larger presenting less of a demand on lenses than adding more smaller pixels to an existing size.
The statement quoted seems to me to assume that we expect the sensor to set the standard for adequate resolution, rather than the displayed work (print, in particular). Some make really big prints and need everything they can get, but they are very much the exception.
Professionals made large prints from medium and large-format cameras--prints that were sharp enough to sustain the illusion of endless detail even when viewed closely (which is my personal standard). The lenses they used were just like the ones you sold. But those lenses could make big prints then and they can now.
But when we view an image on our monitors at 100%, the typical 100-pixel/inch monitor represents much greater enlargement than any print most of us are ever likely to make. My 645z, viewed at 100 pixels/inch, makes images over seven feet wide--a print that would be something like 60x84 inches. Those who make prints that large are the vanishing minority. But if I can make an image sufficiently sharp at that resolution, that would seem to me to exceed the requirements implied by any of my use cases. I own two newly designed lenses for the 645z and many older lenses designed in the 90's for the film version of that camera, but new enough to be computer-optimized with low-dispersion glass (in some cases) and with autofocus. In nearly every case, I can attain my standard even at 100% on my monitor, if my technique is optimal. Of course, these lenses were intended for professional use.
But stuff those same 50 MP onto an APS-C sensor, and one's lenses now have to render detail onto the sensor one fourth the size mine do. Or, we could make the same prints we have always made and even those old lenses work great.
This is quite similar to the concept of the audiophile buying music to demonstrate his system. Buying lenses to make images look good at 100% on a monitor is, well, masturbatory. Making images and displaying them meaningfully is, for nearly everyone, utterly unlike that 100% monitor representation.
Of course, focus-plane performance requires that everything measured to that level of sharpness be exactly in the focus plane. Even within the depth-of-field window, stuff outside the focus plane will be less sharp. That is, again, a matter of technique.
Rick "who only has one lens for the 645--of 15--that cost him more than $700" Denney