You don't understand me, IN THE PAST I had a humidity problem yes! and that SHOW ME! that MYLAR can HOLD the mold spores, is too obvious is plastic, in a case of HUMIDITY plastic mylar can HOLD more the humidity, mold spores. By example brand new records are sell in Shrinks (cheap plastic not mylar of course), I had some still 70s records in shrink in the same boat with the humidity problem and ALSO holds the humidity INSIDE, so again, any plastic even MYLAR can HOLDS humidity IF humidity exist of course.
Like you say, IN THE CASE OF HAVE A HUMIDITY PROBLEM, I'm not saying that just for add Mylar we can add or add more the humidity. I just saying that I bet if I will not have any plastic on my stuff the damage will not be high, thanks to mylar sleeves the molds hold more spores inside the bags.
NOW, I fix my humidity problem, I have a room with a dehumidifier keeping the humidity between 45% to 55%. Also I add Silica Gel to my boxes and of course I REMOVE all mylar sleeves that had tons of mold spores.
Oh by the way, all sleeves that I use have flap with adhesive strip, now I switch to NON FLAP sleeves, I read that too, sealed sleeves can increase the humidity due not can breath.
Yes yes, I know a lot here think that I'm stupid or a newbie but think on that, in some way make sense that plastic can holds more humidity or can cause other issues on paper, make sense storage paper with paper, like posters collectors do, posters collectors use intervaling paper to storage , the thing with comics or records collectors is we love SHOW the cover arts so of course we need plastics.
See this that company offer that kind of thing of storage comics, again, no mylar, no plastic, pure acid free paper!
Why I feel that some people feel offended by all this? is just a discussion about plastic vs paper to storage/archive valuable collectable items.