The money will have to come from increases in taxes, and it will be difficult to not raise taxes on the middle class. Elizabeth Warren just released a plan that comes close to not doing so, but it's unclear if she will actually raise enough revenue. Also, due to the incidence of payroll taxes, it may cause a tax increase for the middle class anyways. As most labour economists will tell you, labour supply is very inelastic. If a new payroll tax is introduced, decreasing workers' wages, most workers will simply continue to work the same amount of hours or only work slightly less. This means that this is an efficient revenue source... but the tax incidence falls almost entirely on workers.
4) M4A is no better than a public option. It could be argued that these downsides would be worth it if M4A was truly the best option we have, but this is clearly not the case. Multi-payer system, such as in Germany and Switzerland, routinely meet and exceed the performance of single-payer systems, such as the UK and Canada.
M4A is simply not worth it, politically, fiscally, or electorally.