If you want to use the word "back", as in your "step back", in the current form you have the sentence:
In the 1990s CDs were the most popular format for music, while in 2010s they ..... digital recordings.
You could say took a back seat to. But you also say you'd like something in a slightly higher register, something more formal or official.
In that case, you can consider:
In the 1990s CDs were the most popular format for music, while in 2010s they were superceded by digital recordings.
You may also use surpassed in place of superceded, but while the former suggests digital recordings became more popular, it lacks the sense of complete displacement, or obsolescence, of the latter.
Or, if you want to get even "fancier":
In the 1990s CDs were the most popular format for music, while in 2010s they abdicated their throne to digital recordings.
Note that this word or phrasing is much less likely to be understood by the man in the street, but it does add some personality to the language. It sounds like something a journalist would add to a human interest article, or something that would appear in a magazine like the New Yorker or The Atlantic.