easyJet already fly SOU-GVA with the 319/320. If they wanted to fly shorter sectors from SOU to EDI, GLA, BFS, AMS etc then there is no technical reason as to why they couldn’t have done it already. That they haven’t is clear, but the reasons less so.
The comments about winter temperatures are also a bit of a false trail. The bigger impact on performance is a wet runway, which you’re more likely to get in winter.