Anon – we didn’t “come up” with an excuse for yom tov sheni. It’s in the gemara, well after the calendar was established.
And there are reasons we can understand; we will use aidim once again when moshiach comes(although then there won’t be yidden outside eretz yisroel…still it will take too long to reach far away parts, as EY’s borders will increase by a lot.
And there’s a ton of reasons al ki kabalah for it, as well.
Did you spend time in yeshiva outside of high school?