I agree on Medicare. But it's plausible that the Supreme Court could find government meddling on Social Security unconstitutional, because similar decisions have already been made in other countries.
In Finland, the mandatory pension system is similar to Social Security, except that it aims to replace ~60% of earned income without any limits. Pensions based on past contributions have been established as constitutionally protected private property. If the system seems to be out of balance, the basic tools are raising the contribution rates without increasing the benefits and increasing the retirement age for younger generations. Any changes that would substantially lower the pensions that have already been earned would have to go through the same process as constitutional amendments. (There is also a third tool: increasing the tax rates for all retirement income, regardless of the source. But that is understandably unpopular.)