How can we be sure SP500 will continue to grow with 7% per year in the future?

Im afraid over long periods of time, like 10-20-30 years, it will only grow with numbers like 3-4%. Meaning inflation will almost eat it all up.

How do you go about this worry? Thought hits me sometimes and whenever it does i always think i got to invest in other stuff aswell, but there really is not anything good. Maybe pay of mortage and stuff like that