It's not such a no-brainer as people think.
easyJet will have done a very detailed economic comparison and as Ray Webster mentioned the Airbus has a risk element, but if the price is right they will go for it. They effectively operate 2 types already the 737-300 and -700 - the commonality in spares isn't as great as people think. Also with the development of new bases, easyJet could have ones dedicated to the 737-700 and others to the A319. So while the Chief Pilot may be a Boeing man, the decision will be based on financial numbers.
Also, the A320 family is already used by other low-cost airlines, eg. JetBlue (A320s); GermanWings (Eurowings's A319s); GoodJet (A320s) and don't forget the charter airlines (Monarch etc with the A321). The charter airlines were and still are low-cost airlines.