@Simon operators are more than happy to support it for foreign phones or networks, till recently they got to charge what they liked for the service and even now they charge far beyond cost. There's no way to hide profiteering in this compulsory national roaming, of course they don't want to play.
The argument against seems to be little more than the networks don't want to do it and government won't be able to compel them or have any clue about doing it right. It's a fair observation, gov IS incompetent and the networks are experts at maintaining profits by fair means or foul. Not hard to guess which will get what they want in that contest.