We have to remember something else, especially with pop music--the engineers mixed these tracks to make a hit record, not to appease audiophiles. In hindsight and now on better equipment, I'm sure many had wished they recorded those tracks just a little bit better.
Thing is, once they get into "fixing" things or worse, remixing, they are taking away the essence of those original releases. A few nudges or some cleanup here and there isn't a bad thing, but if the sound drastically changes, the tunes no longer sound like listeners remember them, warts and all. Sometimes it's even in the mastering--a few changes to a "dead" sounding release might be clarified if the mastering is improved.
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