Not necessarily, all states have the right to self-determination. Under the provisions of international law the Scots could hold their own referendum and if they voted for independence then Westminster would have to respect it. But then again, the SNP's logic isn't really for independence, it does after all want Scotland to be a member of the EU, which at this stage would involve a commitment to joining the single currency, and the Eurozone is well on its way to becoming a fully fledged federal state.
Scottish voters / On a choice between a Scotland...
In the EU and not part of the UK: 37%
Part of the UK and not in the EU: 46%
It would definitely be in the 48% best interest for Scotland to go Indy AND for them to offer passports to any UK passport holder who wanted the new Scottish passport.
Scotland would be at a HUGE advantage to take all the EU City work off London without the need of it leaving this Island.
Its in everyones best interest for Scotland to go Indy otherwise the London work will go abroad.
The basis for a second independence referendum is, in the eyes of many eligible voters, a fairly narrow No vote in 2014 being undermined by Brexit. Arguing that an independent Scotland would struggle economically is not all that relevant to the those calls for a second referendum to be held on that basis.