The simple fact is that a single aircraft can do a transatlantic roundtrip in a 24-hour period, but schedule it six or seven days a week and something will go wrong, trashing your reputation. That's what happened to People Express (remember them?)
And Highland Express who leased in a knackered old 747-100 which spent more time tech than it did in the air. (You can still buy their model aircraft on ebay for around a tenner!)
By what's been said so far it seems that lowcost is down to reliability and performance. It seems just one hiccup will send your reputation to the dogs. So what's the answer to such an operation? A one horse outfit doesn't work. Stby a/c are too expensive. An example that works after the demise of Eos, Silverjet and Maxjet, even though it's J class only is BA's operation out of LCY with 2 reliable A318's. They plan ahead and cut back to 1 service where necessary and the spare aircraft goes in for maintenance. Open Skies seems to be working after faltering at the start with old 757s.