Most likely the relevant rule against perpetuities has a limit of X years or death of a named person + 20 years. And the X is known, but likely to be less than the longest life + 20 years.
The traditional rule against perpetuities is the life of someone alive when the trust is created plus 21 years. That is also the rule in New York, in section 9-1.1 of the Estates, Powers and Trusts Law. SPY’s lawyers probably chose 20 years to be on the safe side.