Speaking only of grammar, meaning, and word choice:
One could write a correct sentence using the word "severity" this way:
"Since the Great Depression of the 1930’s, social programs such as Social Security have been built into the economy to help mitigate the severity of business declines."
"Mitigate" is a verb that means "to make something less bad."
In the phrase as you've written it, the skeleton is "avert business declines." You can add an adjective to answer the question "what kind of business declines." "...avert prolonged business declines..." "...avert deep business declines..." "...avert severe business declines..." In each case, "avert" means that something stops them from happening. The business decline didn't happen. Something averted it.
There are also noun forms. We can speak of "the length of a business decline," "the depth of a business decline," "the severity of a business decline."
We can write a verb that tells about some effect on length, depth, or severity. "reduce the length of a business decline," "decrease the depth of a business decline," and "mitigate" or "soften" or "minimize the severity of a business decline."