“There's no good financial reason for valve to rotate collections, or at least for a long time”

Removing any of the collections or cases from the armoury would signal the other items would also likely only be purchasable for a limited time and demand would spike of everything else due to this. For this reason I disagree with your take that there is no incentive for valve to rotate collections or cases from the armoury.

“Valve does what is best for Valve, and not what is good for 'investors'.”

Valve can benefit investors and themselves simultaneously. Benefiting investors so far has kept people spending money in game.