The reason UAL( Boeing-Lockheed) went Russian is because of cost not because they lack the technological know how.
They already have a design in place in case the Russians cut off supply. But it will just be far more expensive.
SpaceX already has heavy lift capability at far cheaper prices than UAL using Russian engines. UAL and their supporters deride SpaceX designs for being dated but what does it matter if it is overall much cheaper and gets the job done?
Certainly. People who built the engine for STS can build a much smaller engine. Or could, since many of them have retired, and some are dead. Do we have enough engineers with experience in design of rocket engines with cryogenic fuels? I specialize in RF designs, for example; a control loop for a LOX pump would be beyond my practical skills. (I can build it, but I can't guarantee that it will work right on a real rocket.)
They already have a design in place in case the Russians cut off supply. But it will just be far more expensive.
Expensive is just one aspect of the problem. Compatibility is another. Mounting, fuel, pipes, valves, weight and center of mass, new vibration, new resonances, new control systems - all that needs to be simulated, measured, and flight-tested. That's where you burn time. As an example, the firmware for the platform may require mathematical proof of correctness.
UAL and their supporters deride SpaceX designs for being dated but what does it matter if it is overall much cheaper and gets the job done?
I can certainly understand why UAL says that. However I'm afraid that their position has little to do with the needs of the country.