Huge risk pools for health insurance seem to work, so why not support the biggest of all - just one pool to spread out that risk? And why not support the most efficient system for administration? Something based on Medicare is looking very fine:
“Private insurers take, on average, 13% of premium dollars for overhead and profit. Overhead/profits are even higher, about 30%, in big managed care plans like U.S. Healthcare. In contrast, overhead consumes less than 2% of funds in the fee-for-service Medicare program, and less than 1% in Canada’s program.”
http://www.pnhp.org/facts/why_the_us_needs_a_single_payer_health_system.php