Very true.
But it wouldn’t stop Easyjet trying some inbound flying from European bases to SOU. Enough that it might act as a deterrent to people living in Southampton/Portsmouth and thinking of using BOH instead of LGW. Thus keeping business in EasyJet.
However, EasyJet do seem to serve two different markets between SOU & BOH if you take GVA as the example, so perhaps the catchments are less overlapping than we thought?