Isn't it harder to judge the quality of modern lenses, since much of the 'quality' comes from in-camera compensation, rather than outright optical quality? Whereas, when using older lenses you can be assured there are no hidden shenanigans. As long as you're happy with the result I guess it doesn't really matter 
No. Modern lenses, even cheap ones, are great because we have better engineering and manufacturing than in the past. Modern optical design is computer aided, allowing the design of complex distortion-reduction aspherical designs, and modern manufacturing makes it possible to manufacture those affordably.
Yes, many modern cameras (Nikon for sure) apply compensation, too,
but they do that for old lenses, too!!! But the compensation is for things like pincushion distortion, lateral chromatic aberration, etc. It’s not improving the fundamental sharpness of the lens.