Even as a SpaceX shareholder I have to say this the right decision, and it's what a person would expect.

NASDAQ has an incentive to offer early entry into the index (they want SpaceX to list on their exchange), whereas S&P has no incentive for early inclusion; it could only increase risk for them. Even if they /wanted/ to add SpaceX early, that could create a precedent with unpredictable consequences.

Since most shareholders I know plan to hold for another 20 years, waiting a year to get into the S&P average seems fine. Congratulations to S&P for sticking to principle.