The problem with the rent idea, as I understand it, is that congregations are under no legal or fixed obligation to send a certain amount to Watchtower.
That's an important point. There is no penalty (that I am aware of) for congregations that don't send in "enough". The most I've ever heard of is a strongly worded letter.
But letters won't make money be squeezed out from a rock. If the money ain't there, it ain't there.
There's nothing really that the WTS can withhold from its members or its congregations as a consequence of not paying "enough".
Their main method of enforcement is pretty much "we'll be really upset with you, if you don't pay." IF JWs get tired of the empty threats, or figure out there really is nothing to fear, what then?