The way I see it, the current situation with AVIC is more analogous to the one with CSSC after they built out the new halls at Bohai. Unlike shipyard infrastructure for surface combatants, which can be and often is shared with civilian hulls, there's only one reason you need submarine halls. So that meant what we saw was a very clear signal of intent that lots of new submarines were coming over the next decade, even though the new subs themselves did not appear until years later. The fact that old submarines naturally need replacements did not detract from the conclusion that the undersea fleet was set for a major expansion.
Likewise, AVIC is not about to start cranking out civilian airliners from its new facilities. That's what COMAC is for.
The nuclear submarine/Bohai expansion is analogous, I agree -- but the Bohai expansion also did have some caveats (and still has some caveats) that remain unclear similar to the expansion of fighter production facilities.
For example, while we expect an increase in PRC nuclear submarine fleet size by virtue of the Bohai expansion, we do not know what that means for the size of their existing SSK fleet -- would it stay the same, or reduce slightly, or reduce significantly? That is especially the case as they still have modern SSK variants being produced with existing SSK lines either remaining in good shape or even being modernized.
Parallels to the 4.5th gen fleet and new 5th gen introduction are there (albeit imperfect).
The way I see it, an expansion of fighter fleet size is indeed a very reasonable thing to project.
But the prior discussion has been a little bit more specific than just "fighter fleet size may grow to an unknown degree" -- it is instead saying that based on projections of PRC 5th gen production from now to the early 2030s, they may well have their 4.5th gens to become the "oldest" fighters in service by around 2030, and if 5th gen production continues into the mid and late 2030s and if 6th gen production starts in early 2030s, then the question is what happens to the
700 or so 4.5th gens which will all have at another 10 or more years of airframe life on them?
So when we talk about "fighter fleet size expansion" it is specifically in relation to that population of 700 or so 4.5th gens. (J-10C and vanilla J-16) in the 2030 onwards environment.