I thought the salwsman (who also apparently teaches photo classes) was technically right in saying the light from older lenses hits the sensor at an angle but in terms of what the eye can see even on a big blowup it becomes irrelevant. So I posted to get some confirmation - and got it.
I think it's a bigger problem at wide angles. After all, the DA* 200mm f8 released in 2008 appears to be the exact same design as the FA* 200mm f2.8 released in 1993. The angle of light can be a larger issue at wide angles.
Older wide angle lenses designed for film use are not terribly wide on a DSLR anyway. Supply and demand means the handful of good old lenses that are wide enough, like the FA 20mm f2.8, cost more than a DA 21mm f3.2.
Another DSLR lens issue is "digital" coatings. The sensor can bounce back reflections that cause issues with some lenses. The newer coatings and designs minimize this.
Clearly, most of the time the old lenses work fine, or no one would use them. The corner performance is way better on a DSLR because the DSLR only sees the center crop of the lens.