If you remove the cap on contribution and/or means test the receipt then that pretense is gone. Politically, it becomes explicitly a welfare system for people that did not save for retirement and penalizes those that do save. Once Social Security is perceived as "unearned" it becomes an acceptable target for reduction or elimination to the population at large.
Social Security is in fact a welfare tax and no one is entitled to receive it (see: https://en.wikipedia.org/wiki/Flemming_v._Nestor) but its political viability is dependent on the popular perception to the contrary.
