I'm all for single payer in the US but this diagram is a bit misleading.
- There's still a program receiving government funding (e.g. Medicare).
- There's still admin and billing (for the government program).
What I assume you're really gutting are profits and shareholders for insurance companies. (Good, because healthcare in my opinion should not be a profit driven business in any respect.)
What I fear, however, is who is in power at any given time might change the care you receive if such a system isn't setup with safeguards and ironclad mandates.
For instance, Republicans would absolutely attempt, through legislation, executive order, and the courts, to implement an effective federal ban on abortion or healthcare for trans and LGBTQ groups by changing how/if a single payer system would cover these services.
I would also be worried about the public availability of coverage data such that lists of frequent providers for these services are easily obtained and become a tool for harassment by religious zealots.
Or, imagine an anti-vaxxer put in charge of the program during the next pandemic.
How do other countries deal with these issues? Or, have politics become so broken in the US that this is a somewhat uniquely American problem?