For decades there wasn't a social security or medicare fund. What was there was taxes. Workers paid taxes, and those taxes went to retired people. There was no fund. Then people freaked out about the boomers and what that would do to retirement. So they created a fund- they started collecting more than they needed, invested it in government debt so it would grow, and used that to get over the bump.
We can easily solve the "solvency" crisis by simply raising taxes. Remove the "cap" that says people are only subject to the social security tax on the first $160,200 of their income and the "solvency crisis" completely disappears. In fact if we completely removed the cap, rather than just raising it, we could afford to pay out more in social security while still extending the life of the "surplus" fund for almost a century.