No, an A321 couldn't do the North Atlantic, at least not with the kind of pax loads that easy would need to make it viable. I think its a bit generous to even say that they cornered the short haul market. There was indeed a perception that the low cost carriers were the only cheap way to fly in Europe, but that really only persisted as long as it did because it went unchallenged for so long. There was still a strong demand for the traditional full service short haul and the lo-cos were mainly tapping a hitherto unserved market. These days everywhere you look is an advert for BA or bmi offering low full service fares and the public have become acutely aware that not only are these fares competitive with many of the lo-cos, they often represent better value for money with the increased service and support of the traditional carriers.
On the subject of long haul, I firmly believe that low cost in the easy style is a non-starter. People tolerate it in Europe becuase the flight is short and they can eat at either end. It's little different from getting on a bus or train. Spending 8 hours on a flight is different. Even the most price sensitive holiday maker expects a meal or two on a transatlantic charter flight and I bet they don't want to fight for their seats as well. Once you start introducing these things at easy their philosophy of rapid turnarounds, minimal cabin service and high aircraft utilisation takes a hit. Plus, if easy wanted to start transatlantic ops they couldn't have picked a worse time with huge overcapacity in the market and vicious price war well under way. Easy make there money by selling a few seats very cheaply, most in the middle price bracket and some very expensively. They might attract some customers with the very low fares, but I'd be surprised if many of the higer fares sold when theres so much capacity going so cheaply on the existing carriers, but its those fares that allow the route to break even.