It is a difficult one! In the case of boats, they are forced to use facilities only because of the existence of commercial traffic. So it seems underhand to now ask them to pay for the service that they are being told they must use.
Pretty much the same for GA. Without the huge amounts of controlled and mil airspace we wouldn't be channelled around at such a low level through such small gaps as to make LARS virtually a necessity.
Mode S is maybe a way forward? Surely it is not beyond the ability of a cottage engineer to come up with a simple and cheap TCAS like display linked to the Mode S transponder. If we are all going to have to have Mode S, then why not make it useful?