I'm not sure if it's economically realistic but it seems politically realistic.
The right should be fine with the idea of a government agency being less tax dependent and I'm not sure what problem the left would have with it. The corporate interests should also have no problem with it because it arguably gives them slightly more control (ATCs being less accountable to the .gov for funding them makes them more accountable to customers). I'm not familiar with how ATCs work but on the surface it seems like it could be a win-win-win.
That's one potential issue right there – would this arrangement put airlines or airports into a position to squeeze concessions out of ATCs that might reduce safety? At the very least, minimum ATC pay would need to be federally regulated.
https://www.iata.org/whatwedo/ops-infra/air-traffic-manageme...
Not sufficient to run cost free though it seems.