I agree.
I might start in the same place as Warren: the US currently operates a very expensive hybrid private-public healthcare system, without the results to justify it.
And there are some benefits to a government supplied model.
I also don't think it is quite correct to describe Warren's service as ONLY a $20.5 billion increase in spending, without ALSO recognising that there would be an offsetting reduction in private health expenditure. [Different payers and savers, admittedly.]
But it's not like government-only healthcare systems are flawless either. You want everyone to have access, but there are several ways to pay for it. When you use taxation to pay for healthcare, you lose the connection between customers and service providers. With the result that suppliers make things cosy for themselves.
In Democratic shoes, I'd be building on top of Obamacare, just adding a public option. Then using government bulk buying to reduce costs.
I just don't think the Dems will have the votes to do what Warren wants.