If we take in X dollars for FICA and we spend x on SS leaving y dollars that gets spent on the general fund, which costs Y, paid for by taxes and fees: z:
X = x + y.
Y = z + y.
On the current course, x will grow larger than X, y will go negative and we will have to dip into general funds to pay for X. By raising the retirement age, X will grow (revenues will increase for FICA as people will work longer paying) and cover the increase in x (the cost of SS). y will stay positive, FICA contributing to the general fund.