"Why else would they have it, and why else would those that do not have go without?"
I already told you - the people working to get it later are included in the group. It's not just old people.
"Voters skew older, and providing healthcare to the poor at least equivalent to Medicare is not a priority."
It's not a priority because the poor are a small voting segment. The voting segment concerned with Medicare include the elderly and those who are working and thinking about their future retirement, which is a huge segment.
"Medicare premiums get more and more means tested every year."
That's not exactly means-tested. You get Medicare if your old enough, then yes you have premiums and deductibles since this is insurance after all. You even have contribution requirements for things like part A. The permium cost can vary based on income, but that doesn't affect the actual coverage or eligibility.
Yeah, money is fungible. The laws are pretty clear on the funding sources and amounts for the entitlement programs, unlike with welfare programs that are more open-ended and less predictable.
Bend points are not means testing. You get your benefits regardless of your situation. The insurance just pays out based on a formula to adjust to cover the more basic expenses. You can see this intent in the way the formula for payouts is designed as a replacing percent of income as well as in the tax cap.
Stuff like Medicare and Social Security are not welfare programs that are means tested. The costs and benefits can vary based on contributions and income but you are entitled to the benefit. These programs are not powerful just because of the old people. The workers expecting these benefits in the future are a huge source of the political power. It's the money and the fact that people have been paying in with an expecting of benefits that's the difference.