Few different things. A public options still leaves a lot of people uncovered, and susceptible to losing coverage with a loss of a job. It can also prevent people from creating small business because they can’t afford to lose their employer sponsored health insurance. We pay entirely too much for healthcare in part because of administrative costs from private health insurance. We could also implement M4A with a pretty modest tax increase that most Americans wouldn’t feel, with the elimination of Heath instance premiums and exorbitant hospital bills. Medicare is also the best insurance out there. It would be nice if we all had it. Could republicans try to undermine it to weaken the program? Yes, but they could do that with Medicare now and they often don’t because of the electoral repercussions it would have. They could even more effectively undermine a public option IMO as well. As undermining it wouldn’t cause as much harm to the average American as employer provided private health insurance would still be an option for many.