Means testing is a good idea.
For Social Security, too, in my opinion.
The reason you will never see it proposed is because delivering care and help to those who can't afford it isn't the real goal. Political power is, and there aren't enough needy people in the U.S. for that.
The number of uninsured in the U.S. runs about 18 percent of those under 65 (last time I checked.) This also means about 82 percent of those under 65 are insured under the present system.
Medicaid was designed to provide medical care to those who can't afford it, but like I said, there aren't enough recipients in a targeted system to get the kind of power over the citizen the socialists want.