The thing is, if the Lords want to step sending their taxes to their Dukes, they can switch duchy easily. Those who stay make a conscious choice to keep their allegiance to an exiled Duke (and, frankly, should be banned). All the gold he gets is either from his own region or from people who want to send him gold.
Even before the change, I believe you received your tax in gold as long as you were within your own Duchy, right? All the new system does is that it doesn't restrict the exlie's freedom of movement so much.
