Umm.. are you forgetting the C919 is preparing for flight testing right this year--in 2017. If they were to start now, from 2018-2025 it is 8 years to design, manufacture and flight test domestic subsystems that Chinese companies are already mass producing for the Y-20A. Yes, all of them have already been developed by Chinese companies for the Y-20 including avionics, flight control systems, hydraulics, landing gear, environmental control, electric and power systems and so on. So unless you think that cabin interiors or seating represents a major design challenge for the C919 that time frame is more than enough to get the job done. This is assuming that COMAC hasn't already put in place a Plan B for developing domestic alternatives for the foreign gear in the C919 by now, which would be very surprising to me, given the strategic importance of this program since its inception.
You do realize that the most risky part is always the engine, which also happens to be the component most vulnerable to development delays. And guess what, assuming the original news is accurate, the prototype of the domestic engine will be completed this year and 8 years is enough to complete all flight testing. So yes, a domestic version could be made ready well before 2030-40
The question here isn't if they can, but whether they will. COMAC may want to focus all resources on readying the commercial version for service entry, particularly if there are major problems encountered during flight testing. That, I believe, is the thing that can actually delay the military version.