Loaning is in the first place about abundance.
The ability to loan is a measure of abundance.
It seems the purpose of shemittat kesafim is to stop loan activity as we approach the year of shemitta, to stop the investment in new ventures so that the society can invest in managing the transition across the years of shemitta.
Hillel needed to institute the prozbul to deal with the problem of shemittat kesafim because neither he nor the entire people understood the meaning of shnat shemitta. The people of Israel did not properly understand the meaning of sabbatical, of the nation taking off for a whole year from the tasks of economic endeavor.
The people of Israel did not understand that the mandate of shnat shemitta was to welcome scarcity as a counter-weight to the build-up of abundance which can beset a nation.