Older lens on new camera question

[ranchcutter]

I bought a Nikon D50 with the 18-55 kit lens and a Sigma 70-200 f2.8 about eight years ago. I recently bought a Nikon D7100. These lenses on the D50 produced nice, sharp shots, but on the D7100, the shots look terrible. The sharpness just isn't there. I've had a professional photographer tell me that the much larger mp camera (D7100) will bring out the flaws in the older lens, thereby explaining the way they look. Then I saw where someone has said in a thread in another forum that "No lens will become worse with a higher resolution sensor camera". If that's the case, I am truly not liking the D7100. So, now I'm confused. I've spent countless hours the past month in researching lens in thinking that I need to buy something newer. Help please!!! Thanks!!!