[mst]

I also think that the worry for dust due to the extending construction should not be there, lens rentals showed the impressive sealing measures. And non-extending 70-200 lenses pump air around while focussing too, so also can show dust inside after use.

"Impressive sealing" does not help against one very basic fact: any lens that changes its length, pumps air in and out (unlike non-extending lenses). That is not possible without occasionally pumping small particles in and out, too.

Any extending lens design I know will collect some dust over the years. Some to a lesser extent, some very visibly (the 100-400 L IS for example is notorious for accumulating dust behind the front lens group).

From my experience: any extending zoom lens, that has been used (and not been sitting on a shelf) will show some dust inside. If you can't see any, you simply haven't looked hard enough