The Torah says that Passover must be observed "for your generations" (Exodus 12:14), and the Mekhilta immediately spots a potential loophole. The Hebrew word for "generations" is "dorot," and in rabbinic interpretation, the minimum plural is two. So could "for your generations" mean just two generations, after which the obligation expires?
The Torah anticipated this reading and closed the loophole with a second phrase: "as an everlasting statute shall you celebrate it." The word "everlasting" (olam) overrides any minimalist interpretation of "generations." Passover is not a temporary commemoration that expires after two cycles. It is permanent, binding on every generation of Israel from Sinai until the end of time.
This kind of interpretive move, finding and resolving apparent ambiguities in the Torah's language, is characteristic of the Mekhilta's approach to law. The rabbis did not assume that the Torah's meaning was self-evident. They probed every word, tested every phrase against its minimum possible reading, and then showed how the Torah itself provided the answer. The question about two generations versus all generations is not frivolous. It is rigorous legal thinking applied to sacred text.
The result is a commandment with no expiration date. Every Seder table in history, from the first Passover in Egypt to the one you attended last year, sits within the scope of this verse. The Mekhilta ensured that no future generation could argue it had been released from the obligation. "Everlasting" means everlasting.