I've owned the same 100/400 since 1999 -- never had a dust issue or mold, or dirt, or moisture. I take very good care of my equipment, shoot a lot of wildlife and travel, and shot sports for much of that time as well. It gets a lot of use. I've heard this "dust pump' a lot, but not experienced it at all with this lens ...
My work is primarily in USA, desert (dry/dusty), mountains (cold), Southeast (humid, hot) and pretty much every climate. Maybe I got a "tight one" ... in fact, I'm looking at the new 100-400 MkII and wondering if I really need it. Altho' with a fourteen years old lens, the new Mk II is whispering in my ear -- especially the IS x 4 and the rotate focus.
Altho' I've used this lens all these years (hmm, what else?) I'm not a fan of the push-pull style - and would prefer the rotating focus ring like most other Canon lenses.