@lain I think that that is due to an ever changing philosophy of best practices in musical performance. In the western classical tradition the best practices change every generation. For years, Glenn Gould's performances of Bach were the gold standard. Even though they were all played on an instrument that did not exists in Bach's time. Now, more time period authentic performances are appreciated. People going as far as to prefer instruments from that time period. Soon enough a new philosophy will dominate how classical music is played and interpreted.

I'd argue that there are at least a few covers that surpass the originals. "Hurt" by NiN is a perfect example where Trent Renzor acknowledged Johnny Cash's version as superior.

As much as I hate the idea of subjectivism, there is a level of subjectiveness in music. One thing sounds good to one person, another thing sounds good to another person.