This question shows the problem with the bill (along with the fact that it'd be less efficient):

... if you have a system with supplemental coverage, there will be some degree of inequality. Certain people will have a more robust health care access, if they have a job with really good supplemental insurance, versus someone who would be only able to rely on your new baseline plan.

So why preserve even a limited degree of inequality versus the argument the Medicare-for-all folks would make, that a version like what Bernie Sanders has proposed would lead to a more truly egalitarian health care system? Because you’ve designed a program that does still maintain some degree of disparity...