I didn't know the answer but never would have guessed what it was:

What was the first classic rock song to sell 10 million copies?

https://faroutmagazine.co.uk/what-was-the-first-classic-rock-song-to-sell-10-million-copies/

"... However, it wasn’t until 1991 when German rock outfit Scorpions released their Cold War anthem ‘Wind of Change’ that a true classic rock song was able to surpass 10million sales, with the song overall managing to acquire 14 million physical sales since its release.

Interestingly, one other possible contender for this would be The Knack’s 1979 hit ‘My Sharona’, which despite being released before the Scorpions’ hit by a dozen years, didn’t actually manage to pass 10 million sales until 2010, ..."