I bet this is an on-purpose move by Valve, and I view this as a sane action. [1]
Having a game where some players only play in order to win money is, for sure, a no go. If the game is fun, then players will keep on playing it. It may also keep some money thirsty (sometimes very toxic) people at the gates.
It is also smoothing players' frustration and shopping-spree habits in order to obtain a rare item. If you have the ability to trade N rare items for another rare item then you quite surely may obtain any cosmetic item you want for a much lower investment (less boxes to open). The 'grey market' will adapt to this new value.
That's also a lesson on how a closed economy (and open ones too, to some extent) can collapse based on a single actor controlling the rules. That's fair to learn.
[1] EDIT: and probably a preemptive protection for any future legal threat (as some countries tend to prohibit money gambling in games)