For so long as the Scottish Parliament has the support of the people, a democratic UK government will not pose a threat. Law alone will not save the Scottish autonomy from a UK government that is other than democratic.
There is a lot to lose. First, if law that entrenches Scottish autonomy can be achieved at all, what is it to protect? Does it protect the existence of the Scottish Parliament and government, but not any specific power?
That would seem to leave the legal possibility of the Scottish institutions continuing in existence but deprived of their powers.
Does it mean protection of all existing powers such that the Sewel Convention is made law? That would make it difficult for the UK Parliament to legislate for Scotland even where it’s desirable.
It would also require a much more detailed description of the Scottish Parliament’s powers than is provided. A more detailed description would necessarily be more restrictive.
Second, the danger is that if the Scottish institutions are entrenched in law in the form recommended by the Smith Commission, such a law could become a significant obstacle to progress beyond the recommendations of the Smith Commission. The mere fact of entrenchment may well preserve the changes in aspic.
Third, unless nationalists are in a position to dictate the entrenching law, restrictions on the Scottish institutions may well be entrenched alongside the protection of the Scottish institutions.
It’s not impossible to imagine a constitutional restriction on independence referendums, or qualified majorities for transfers of power.
The UK’s present unwritten constitution provides flexibility for Scotland to develop its autonomy whenever the necessary popular and political pressure can be brought to bear, without extraneous legal obstacles.
A written constitution may well confine Scotland within Lord Smith’s political compromise, whatever the requirements or experience of the future.