Given how much money GAL are spending on airport improvements with a lot more to come, i find it highly unlikely they would have done this if one of the airports major airlines was likely to leave, many improvements have already taken place to the North terminal, more are happening now, including behind the scenes and as SRG points out more are planned, including to the BA check ins.

I would love to see more services from Gatwick, i will go out of my way to fly from there even if the fares are more, just to avoid having to get to LHR.