I think this is a case of seeing what you want to see. According to the Trustees Report, SS is only in deficit if you don't factor in interest income. [1] It's projected to deplete its trust fund in 2034. Please resist the urge to argue that the trust fund doesn't exist. That 2034 date can easily be extended through minor tweaks, check out any of the ideas from the late Robert M. Ball.
The flat tax is regressive. Regressive is bad because the value of money is roughly logarithmic as it increases. Any plans to make the flat tax "less regressive" is an argument against a flat tax. A simplified tax code is a good idea. It is quite possible to have a simpler tax code that is more progressive. Yet I find that flat tax proponents tend not to be in favor of that.
The flat tax is regressive. Regressive is bad because the value of money is roughly logarithmic as it increases. Any plans to make the flat tax "less regressive" is an argument against a flat tax. A simplified tax code is a good idea. It is quite possible to have a simpler tax code that is more progressive. Yet I find that flat tax proponents tend not to be in favor of that.
[1] https://www.ssa.gov/oact/trsum/