Please persuade me why Medicare ought not be means tested

I must say, on this side of the Atlantic, I know I’m not the only one finding this thread absolutely jaw-dropping:

As for the broader question of communal taxation vs. private provision, there’s another angle to view it from: the taxes I pay now are in part my contribution to the return taxpayers as a whole are entitled to expect on the investment already made in the infrastructure and socioeconomic-politicolegal environment that enables to me to have that comfortable an income.