I think you are underestimating the massive priority the C919 is for the Chinese government right now. If the Western companies pulled out, those components would be redesigned and import substituted by China. Heck, the Russians are doing it, with a much smaller economy and resources than China can throw at it. And no amount of Soviet legacy would help with making modern avionics. China has a much better electronics industry than Russia could ever hope to have.
Soviet legacy
absolutely helps.
There is a continuous track of developing civilian flight subsystems since the dawn of flight to right now. Like,
all of them. No surprises beyond development mistakes that happen to everyone. Same with certification.
Japan also has a much better electronics industry than Russia could ever hope to have - yet it thunderously failed to produce even a constructor of Western parts.
I would only take that bet IF some major provocation took place. One likely started in Beijing.
Even if we assume that provocations are only started by nefarious actors and not the blessed Valinor - as it happened in feb.2022, Beijing won't be exactly notifying every granny, especially granny with daily contact with Western business, about what happens next.
So industry should expect such events to come unexpectedly, or the whole point is lost - it's just a for-profit product otherwise, with no more inbuilt survivability than ZTE phones.
And IMHO the better way out is
both - developing fully domestic C919(if it's possible), as a solution with expected fruits into 2030s.
And developing some sort of C919-21 amalgamation, for a stopgap contingency for later 2020s.
We like it, we hate it, a crysis can develop tomorrow. Not everything is predictable.