You're being a little simplistic here Guv (deliberate?), as you are suggesting that the low cost carrier will always wipe the floor with the full fare one. It's not necessarily true. The promotional fares will certainly get Go noticed and will have the usual effect of growing the market.
The schedule is all important to the key business pax and Go's is OK southbound, but not northbound (where the bigger market is) at the present time. The existing carrier can respond by discounting the off peak flights and keeping the peak flights for the business pax, who are the key to the success or otherwise of the route.
Sadly, we're talking about BA here who will do the usual thing of a) not reacting at all and getting stuffed and b) panicking and opening the key flights so that the business pax will also be able to buy the cheapest fares.
It's their usual response and the reason why they always get thumped. But it doesn't have to be like this if only they would learn.
The reason why they always lose out is not because it's inevitable but because they are commercially inept at shorthaul.