I'm not highly geeky when it comes to resolving power or pixel peeping. I've just had focus issues with older lenses on newer bodies. My old Nikkor 50mm 1.4 would only focus properly in live-view on my D7000. Had a bad back focus issue. Have an older Sigma F2.8 zoom that would never focus at infinity. You can always send the lens/body in for repair/resolution I suppose but at what cost if you are not using it every day? I hear complaints about newer lenses being kinda sterile compared to the images produced by older prime lenses. I love my prime lenses but they are of limited utility when you are working an event and I like being able to zoom/crop when taking landscape shots. Outside of a studio or other controlled setting the newer, better quality zoom lenses just make life so much easier and still produce great images but It's all a matter of your needs and style. Unless you are making a living selling huge fine-art prints I can't imagine there will be a significant difference between the usable output of older primes and newer zooms (pixel peeping aside).
Good shooting!
Good shooting!