One could say Delta are only at Gatwick because of JetBlue. Not to detract from Gatwick at all, but they (DL) have a serious track record of retaliation.
United are snipping LHR flights at present so can’t see them adding another London station if this trend continues.
TATL carriers will always have an easier time getting in to Heathrow with open skies . Gatwick will always see more success in attracting services to the likes of China and India where the number of flights are limited. Not to rule anything out, though.