Now if it would be a custom established by the Supreme Court, then it would have to be nullified by a supreme court that is accepted by more חכמי ישראל. But the Supreme Court in Jerusalem did not establish it. It was a custom started in Babylon by the local people because of the worry of forgetting how to calculate the molad and having to go back to going by the Supreme Court which would have to send witnesses. That is why even outside of Israel any place where witnesses can reach in time never has two days of Yom Tov. For example in Mitzraim. Therefore this law has nothing to do with being outside of Israel. It always depends on where witnesses can reach.
The point here is that the reason the religious world ignores these simple facts is not because they are hard to understand. Rather they simply do not care what the Torah says. Their religiosity is all smoke and mirrors. It is the same reason the Supreme Court thinks it can regulate what you grow in your backyard under the interstate commerce clause. It is not that they are so dense as not to understand the Constitution. It is rather that they do not care what it says.