It's "necessary" in the way that where the remaining routes at STN are profitable, there's surely no reason for them to move elsewhere. I'm not saying it will never happen but can't see much advantage in doing so.
Like it or not... EZY may not have much choice but to turn to STN again if suitable slots cannot be attained at LGW or LTN because as you say - they're at capacity.
By your logic should that mean that FR move all their LTN and LGW routes to STN?