It seems that this 'lack of frequency' is an easyJet issue rather than a Southampton Airport issue. Yesterday easyJet announced a twice weekly Amsterdam to Newcastle service. Hardly the frequency you could expect on such a route and certainly something people would complain about if this was Amsterdam to Southampton.
Another point is that, with no base at Southampton, it will be more challenging to get space freed up on aircraft time elsewhere to fly into the airport - forward planning is the key here. 2025 will be the next opportunity and assuming easyJet is content with Southampton on 1) yield, 2) no cannibalisation from LGW and 3) a low enough airport fee then I can't see them not adding more leisure routes.