The simple answer is that we do not know what happened to these women because we are not told. However, kings in the ancient near east were rarely monogamous and had a harem of women (cf. Esther 2:3 ESV). Even after the King chose a wife, it is probable that he kept a harem of women.
We do not know specifically what happened in the case of Esther, but there is no indication that she altered this practice.