We have 2-day Yomim Tovim so that we will be used to keeping
2 days when Moshiach comes and kiddush hachodesh is
once again performed by Beis Din. However, we may be able to communicate Beis Din’s decisions more quickly now.
That’s the Chasam Sofer SDD linked, but it doesn’t answer for modern communication (except for R”H which is anyhow kept for two days in E.Y. as well).
For that, perhaps Sam’s point is helpful, although I don’t necessarily think we overturn minhagim based on changing circumstances either.