For once we can say, without fear of hyperbole, that this has been a truly momentous week in Scottish politics.

Monday's Edinburgh Agreement set Scotland on course for a legally binding, single question, referendum on independence, likely to take place in the autumn of 2014. At the SNP conference in Perth there was quiet satisfaction at the deal struck with the UK Government, especially the provision for 16 and 17-year-olds to be permitted to vote.

Not many delegates were enthusiasts for having a second question on the ballot paper offering "devolution max" or federalism – though the leader of the party, Alex Salmond, had always insisted that Scots who want neither independence nor the status quo should have a say in the referendum. Clearly they will not.

Salmond sought to appeal to this disenfranchised middle in his conference address yesterday. It was a speech shorn of triumphalism which implicitly recognised the political reality as recorded in the opinion polls: that most Scots do not currently want to leave the United Kingdom. It was audacious, in that it argued that, for Scots to defend the achievements of devolution, they had no alternative but to support independence.

Salmond castigated the "incompetent Lord Snootys" – the leaders of the UK Conservative Party, including David Cameron and the Chancellor, George Osborne – who think they can run Scotland. And he tore into the Scottish Labour Party for supporting Tory calls for cuts to Scotland's cherished social programmes. "And so Labour and Tory – the two great pillars of the Union – united in a death grip programme to sweep away concessionary travel, free prescriptions and education."

Echoing Donald Dewar, whom he name-checked in his speech, Salmond said that independence was "a process, not an event" and that for Scots to complete their home rule journey they would need to say Yes to independence.

But the argument has flaws. Saying that Scotland is being run by a bunch of Lord Snootys who dismiss policemen as plebs and don't like paying their train fares, while good conference knock-about, misses the point. The Scottish Parliament has responsibility for policing in Scotland and transport is also largely devolved. In other words, Scotland already has a substantial measure of self-government over domestic policy.

And where the economy is concerned, the SNP is proposing that Scotland should remain within the sterling zone, and under the influence of the Bank of England's interest rates. So the Lord Snootys of Threadneedle Street would still have a say in Scottish affairs.

Those universal benefits which have been delivered by the Scottish Parliament, by devolution, can surely also be defended by that parliament, within the devolution settlement. There is no reason why Scots necessarily have to leave the UK to protect the achievements of devolution.

The case for independence cannot rest on defending the achievements of devolution alone. It has to convince Scots that leaving the UK, withdrawing from Westminster, setting up an independent army, civil service, revenue and welfare system, will actually be worth the effort. Above all, the economic case for independence needs to be made more convincingly than in Salmond's speech.

Asserting that Scotland would be better run without the Tories isn't enough. Nor is it sufficient merely to list Scotland's natural resources, universities, oil and whisky. These are valuable assets, but they do not amount to an instant economy. We believe that the case can be made, and we wait to hear it spelled out in the next hundred weeks.

Nevertheless, the SNP leader has a point when he warns that, if Scots vote No in 2014, the forward march of devolution could be halted. There is no guarantee that result will get a parliament with greater economic powers. We have little confidence in the vague promises made by David Cameron, and the Unionist leader, Alistair Darling, of a new, improved Holyrood if Scots reject independence. In a real sense, the Unionists have upped the stakes in this referendum by denying supporters of devolution max an opportunity to vote for their preferred option.

Salmond is nothing if not a gambling man, and he has bet the house on the expectation that he can persuade enough Scots to lend him their votes in 2014. It is, as he said, "game on".