And what exactly is Scotland going to be doing to prop up it's economy. England is definitely the commercial heart of the UK. How would Scotland prevent England (or almost any other country) from annexing it through military means or diplomatic manoeuring? Where would Scotlands diplomatic power come from? How would you stop England screwing you over when it comes to currently shared infrastructure? As much as I support Scottish independence ideologically, it doesn't seem like you'd be left in a strong position.
If you understood the UK you would know there is absolutely no chance, zilch, zero, none, nyet chance of England annexing Scotland through military means or diplomatic manoeuvering. It's even less likely than North Korea annexing China. We actually have a strong sense of goodwill between our nations in the British Isles, even between ourselves and Ireland. It literally would not cross their mind.
When it comes to England screwing us over occasionally, infrastructure is shared with the Irish and we don't screw the Irish over in the way you're talking about. That's not to say England wouldn't screw us over at all - they probably would at some point in the distant future in which case we would have a diplomatic dispute about it. It's nothing to vote no about though. I'm sure we would become very, very strong allies in the event of Scottish independence.
Our economy is also far stronger than people commonly understand it to be. Though England is the commercial heart of the UK, Scotland has some of the largest oil reserves in the EU, we have a flourishing tourist industry (which would grow enormously with independence - I have a thousand ideas for it), we have an excellent agricultural industry, we have a multi-billion pound whisky industry (whisky and oil usually go together in these debates in terms of importance). Our economy is more varied than Norway's and Ireland's. There's also the renewable energy if you want that old chestnut, seeing as it was the SNP's intention to make Scotland the "Saudi Arabia of Renewables".