I don't even support Talarico, since his position on Medicare access for all seems short-sighted.
The Coordination of Benefit (COB) process alone would be a giant pain. If Medicare erroneously believes a beneficiary has alternative coverage, like Medicaid, it can require billing the non-existent primary coverage first.
Also, for those with private or employer-sponsored insurance plans, adding Medicare would inevitably allow those insurance plans to shift more costs away from themselves with the expectation Medicare helps pick up the payment. That would further serve to enrich insurance companies at the expense of American taxpayers.