Actions like this can surely only have the effect of reducing donations in future. Because, obviously, if people are donating for something, only for it not to happen, and the money be taken away, then they are not going to contribute as much in future.
What is more, Watchtower surely realises that behaviour like this will reduce future donations. Which must mean that Watchtower has such a desperate need for the cash in the short term that they don't mind jeopardising donation levels in the longer term.
Plus saying that the money will be used for KHs in poorer lands is just gratuitous. How many JWs will actually believe that? A significant number of JWs are going to think to themselves: "is it not bad enough that Watchtower is confiscating money we raised locally for an Assembly Hall? Do they need to lie to us about why they're doing it as well?"
Some people have claimed that Watchtower has lots of money at the moment because of the sale of Brooklyn properties. They claim that Watchtower cutbacks are prudent and not a sign of desperation. Moves like this, confiscating an Assembly Hall fund, do not indicate prudent long term strategy, they indicate measures of last resort, or panic.