No one seems to have mentioned that one of the distinguishing features of a language is the existence of a substantial corpus of written material- literature, but also histories, newspapers, general works- in that language.

Now there is quite a lot of literature in Scots, as there is to a lesser extent in say Lancashire dialect (Tim Bobbin's poems for example), but it is often self- conscious in both cases. The newpapers in both Edinburgh and Preston are published in the national standard English (with occasional use of dialect words in both cases). There are no general works as far as I am aware in either dialect- no one would publish a serious work of history in Scots.

Historical documents are more complicated- Scots seems to have been slower to adopt the developing standard than England, and retained their version of Middle english usage until after the Act of Union.

So perhaps Scots was on the way to developing into a separate language in the 16th-18th Century, but the Union retarded this, and it now hovers uncertainly in Limbo (rather deserted since the Pope cleared out all the Catholic babies).