.
If brood combs are old, pupa silk makes cells smaller.
In general I agree with much of what you say as you are plainly very experienced and know what you are talking about.
However, this one is a myth that is repeated so often it has become accepted as a fact.
Over the course of a season this has some truth to it, but the bees do a great job of cell cleaning each spring (its the fluffyish brown debris you see outside the entrance at first broodnest expansion in spring). They return the cell size to desired parameters every season.
The late and sadly missed Dave Cushman and I had an interesting exchange on this matter several years ago when we described some odd brood combs in our unit. They had been there as long as I could remember inherited from my father. They also had very distinctively wired foundation in them in a grid pattern. Dave identified the frames and wax as both being considerably pre war. I do not recall exactly without digging back but he thought the pattern dated from the 1920's.
The combs were black and hard, yet the bees raised perfectly normal sized brood in them. If they had been brood combs for even half their life, then at the time they were 80 years old so would have been brood combs for 40 years, so maybe 200 generations through them? Should have had tiny cells, but did not. All gone now, combination of a psycho extractor that was my most stupid purchase ever, and the extreme comb cull we now undertake to attempt to rid ourselves of EFB where all combs are dated and allowed only a limited life.
Further proof of the annual clean out came with the use of white Pierco plastic foundation. By season end they were as black and opaque as any old wax brood comb, yet at the spring clean out you could hold them up to the light and the cell bottoms were again translucent.
No Pierco now either, all got rid of as it was not sterilisable.