The point was that they did not need the mirrors anymore to convince the men to continue to raise families. While in mitzrayim, the men were so downtrodden, overworked, and in despair, that they would have just given up if the women had not taken the initiative. Consider that even Amram was ready to give up and divorced Yocheved. He would not have been willing to have Moshe, had Miriam not convinced him. Once they left mitzrayim and had received the Torah, the people were happy to serve Hashem and raise their families without the external stimulus of the mirrors. The women wanted to donate the mirrors to the Mishkan in thanks for what Hashem had done and to acknowledge what the mirrors had done for the people.
Rashi points out on Vayakhel 38:8 that the כיור was designed to make peace between a man and his wife. Thus, it was appropriate that the women donated in memory of having been used to create the families of those who survived the slavery in Egypt.
This is [the meaning of] what is בְּמַרְאֹת הַצֹבְאֹת [lit., the
mirrors of those who set up legions]. From these [the mirrors], the
washstand was made, because its purpose was to make peace between a
man and his wife.
Rav Hirsch says:
It is deeply significant that the vessel of the Sanctuary which was to
represent "the moral keeping holy of ones acts and efforts" קידוש
ידים ורגלים was made out of the women's mirrors. Mirrors are articles
which lay stress on the physical appearence of people being an object
of special consideration. So that it was shown that the physical
sensual side of human beings is not merely not excluded from the
sphere which is to be sanctified by the Mikdash, but that it is the
first and most essential object of this sanctification. After all, at
rock bottom, as Man has complete free will in moral matters, it is
just this side of human nature which is necessary to come under the
influence of the Mikdosh, if the sanctification of life which is aimed
at is to be achieved.
Note: Rav Hirsch points out that
the כיור was made up of the mirrors fitted together almost without any
alteration at all, so that it was recognizable that the basin
consisted actually of mirrors.