Gaon: I am not comparing the chumra of gebrokts with Yom Tov Sheini. I am responding to the OP who named this thread. “minhagim-after-the-original-reason-is-gone”. Keeping Yom Tov Sheini today is based on Minhag Avoseinu B’yadeinu. We have a fixed calendar and know what day is Rosh Chodesh since Hillel’s beis din set the calendar. Therefore the reason behind why in those days they held two days was a true safeik. That reasoning is no longer applicable yet we still hold two days.
Avi K: It is not three days 4 times. Rosh Hashanah is two days for an entirely different reason. Only two days are Yom Tov.