I assume they already are. Not just from explicitly benefit reductions like increasing the retirement age, but also the decreasing purchasing power of a dollar (inflation) outpacing any cost of living adjustments. I still have 35 years to go before I can collect, but I expect the demographics will be so tilted by then, that I will see little benefit from Social Security.
In the sene that SS is a broken pyramid model that depends on short retirements and population growth, yes. However, this is not a problem the US is willing to contemplate as they would removing the payroll cap, which would become a significant national conversation.
Inflation, not so much an issue, since benefits and maximum contributions receive COLAs and theoretically wages are inflating at pace with CPI.
It would make it progressive because those proposals do not lift the benefits cap (most of them don't, at least.) But I do understand what you're saying about the flat percentage of tax taken out per dollar of income.