While some years are slower than others and there's the occasional recession where groth does go negative, long term economic growth is very reliable. In the US, GDP has increased 52 out of the past 60 years, with GDP per capita increasing 2200% in that time period. If you invested in the S&P500 in 1960, you'd have made an inflation adjusted return of over 4000%. Wealth is constantly being created, infrastructure is built faster than it's destroyed, there are scientific and technological innovations, and people become more educated and better skilled, and quality of life generally improves.

It's really difficult for an economy to legitimately go down. That takes like North Korea levels of mismanagement. If your country collapses, the sovereign wealth fund is likely the least of your worries.