Yes, and China has gained such experience in the ARJ21 project, which has been going on now for 20 years. Which part of that project which once used western suppliers have been domesticated? Please name them.
The Chinese airliner projects using western suppliers are already unacceptably late, they have no guaranteed market, and are not competitive. These points all pply to the existing projects. The difference is that with non western suppliers, the projects at least have a chance to come into mass use and replace the Boeing/Airbus duopoly bc decades of work can't be destroyed by the stroke of a pen or tweet from the white house.
They're not unacceptably late for Chinese domestic use, and C919 and C929 will both have a guaranteed market in China.
The problem is that for both of those projects, if you want them to enter service in any form within the foreseeable future (respectively) you will have to use western suppliers. If you want to wait for a secure supplier, then that is fine, but then you are also accepting that you'll have to wait for years and years and years for those suppliers to offer a sufficiently mature product to even begin doing construction of the first prototype airframe, let alone conducting flight tests.
This ignores the fact that you have limited budget. You keep saying that there should be a dual track strategy. Besides the engines, where is track #2? Which projects are working on the non-western avionics, landing gear, wing composites, and so on? You never hear about them because the project with western suppliers sucks up all the time and resources.
CJ-1000A and CJ-2000 are the domestic engines being developed for C919 and C929.
You can bet that domestic alternatives for other foreign subsystems are also being developed, but they take up virtually no public attention given their less flashy and visible role in the aircraft.
Well the West for its part is careful not to source any irreplaceable hi tech from China, even at the cost of important sectors like it's 5G network, and even though China has never done any "entity list" style supplier cutoffs.
That is because they have the luxury of having other alternative suppliers which are able to fulfill requirements even if they are at a higher cost and with slightly reduced performance.
For China, depending on the industry, many of those options do not exist. If they did, then they would have pursued it.
If it seems like everything you are suggesting is "common sense" to you and "why are they being so stupid" -- perhaps consider that it's because the options and the possibilities you are describing are untenable and non-existent.