Angels, apostles and other mythological characters have a longer shelf life. Many of them last thousands of years before being replaced by different mythological creatures, so kids stuck with the names won't be quite at noticeable. 15 years from now the only way Khaleesi can be anything other than an extinct, silly name is if GRRM still has not finished the series.
Those names weren't always thousands of years old, at one time they would have been just as new as Khaleesi is now.
Names have constantly shifted in and out of fashion with parents making their choice on royals and popular local figures.

