You might be able to develop them longer than five days, but then you hit the ethical issues. The entire moral argument for abortions is based on bodily autonomy of women, and that those rights trump the rights of an unborn fetus. But for raising a fetus in an artificial environment there is no body autonomy argument.
Another issue is murkier, and is based on the circumstances. Generally abortions are used by women who did not intend to get pregnant as a last resort form of birth control. When you're performing abortions for scientific purposes it's different, and there are different ethical standards to consider.
One shouldn't trump the other.
There's nowhere near consensus on that issue in society. Also the ethics of abortion wouldn't be the same as the ethics of such experiments; in abortion the rights of the mother are at play, minimally, and there are reasonable discussions about when an embryo obtains rights as well as whose rights reign supreme.
Even if considered legal, given that law != ethics, the balance of rights is not the same as abortion.
I would also expect the scientists to deal with this pretty conservatively because if they push those limits, the likely cries to stop funding, etc, etc, would become much more pronounced and the likelihood of the work hitting a political wall much higher.
http://www.nature.com/news/embryology-policy-revisit-the-14-...
Will be interesting to see the moral and ethical debate on any change here.