The rabbis understood this, and created instead a spiritual core to Shavuot that was text-based and removed from its agricultural underpinnings. Now Shavuot was about being spiritually ready to accept God's Law. This is where the themes of Passover and Shavuot begin to mesh - in order receive the Torah, the Israelites first had to become a free people. The liberation they experience at Passover allows them to then receive the Law at Sinai. Each holiday hangs on the other.
Freedom requires laws to live by, and living by the law requires the freedom to be able to do so.
The Shavuot Anthology . Jewish Publication Society of America, 1975. A useful guide for creating meaningful celebrations and gaining a better understanding of the holiday. The volume contains sections on the history of the holiday and its observance, its representation in art, poetry and prose readings from both the ancient and modern sources, and the music associated with the celebration. Kates, Judith A. and Gail Twersky Reimer.