Southampton is completely different with a catchment that includes southwest London, Portsmouth, Bournemouth, Brighton even. The fact that its runway is short has indirectly assisted BOH in attracting medium to longer range routes with aircraft above 120 seats, including ourselves. If SOU had a runway that was as long as BOH's then i would worry about the ability of BOH to attract carriers.
SOU's success has been down to high frequency, business traffic using smaller aircraft on short haul routes. This is how BOH and SOU have competed effectively with each other, almost complementing each other.
I see the future for SOU as more of the same, and for BOH with less of what SOU has and more longer / medium haul routes with a heavy emphasis on seasonal leisure based traffic and Ryanair might not always be part of that future.