I am sorry but your argument "allowing people to wear the burqa or niqab "opens the door to the possibility that all women might some day have to wear it"  could be applied to abortion, same sex marriage or any law. The argument is that in upholding one person's right to choose their outcome, the government can force others to not only capitulate but personally participate.  Do rational feminists really believe this?
Further, if coverings are signs of oppression then dis-allowing them in public will only ensure the movements of oppressed women will be further restricted.    In fact, they will very likely never leave their homes. When a Quebec adult ELS teacher demanded a woman remove her niqab or leave her classes, the woman left wearing the niqab.  Did the edicts of the government liberate that woman from oppression? No they denied her an education and the possibility that after mixing with Canadians she might gain the confidence and strength to decide to remove her own covering.