I own two businesses in healthcare. I understand what you are saying about seniors' perceptions of Medicare. Medicare is insolvent. It could never be solvent over the course of generations. It is necessarily insolvent due to the aging population of generational cohorts. Larger cohorts will use up all the money leaving no money to pay for smaller cohorts. (In the immediate case, the baby boomers are the large cohort who will use all the money, leaving a deficit for generation X and absolutely NO coverage for Generation Y.
Social Security, Medicare Face Insolvency Over 20 Years, Trustees Report - WSJ
The entire system is based on the younger cohorts paying for the older cohorts. That would only be feasible if either a) every cohort was the same size or b) cohorts became progressively larger. Neither is possible.
It doesn't matter how "excited seniors are." They only way it would have worked properly is if everyone's withholdings from Medicare were set aside for them, rather than robbing peter to pay paul. A universal system is no different, except that the health care options would not be of high quality.
Medicare dictates prices to healthcare providers- many of whom are progressively refusing to take Medicare. Making them take a public option will just force good doctors out of medicine, stunt advances in medical technology and pharmaceutical R&D, and make the practice of medicine a less than lucrative option for anyone but the most bleeding hearts of college graduates.
America is a capitalist economic system. It is NOT a socialist system.