[AnthonyMouse]

A significant fraction of the historical rate of return on capital is attributable to the combination of population growth and inflation. Without those the size of the fund you would need to cover a material fraction of government spending would be implausibly large.

Sovereign wealth funds also aren't all that different economically from taxation, in the sense that the profits from what the fund is invested in go to funding government programs rather than being reinvested in private enterprise. If the fund was large enough to be relevant, that would become an issue for the economy.