An independent foundation, unlike a DB plan, does not need to consider the correlation between plan sponsor financial performance and the performance of the portfolio.
Foundations liquidity needs are defined by the spending rule...
It does fluctuate over time because a fixed % spending of fluctuating assets does vary but in essence, the foundations are not required by any defined liability to provide any other funds than the required 5%...
If it's purpose it to increase the amount of grant-making, then yes, the liquidity needs should increase year after year but if it's purpose is to provide a constant stream of grants year after year adjusted for inflation, then it should remain relatively constant on a real $ basis...