I think at least with the current schedule easyJet have on offer, there is space for both airlines. Loganair only carried about 50% of pre-covid numbers this August (compared to August 2019). I understand that the route most likely won’t get back to its former numbers and that it’s winter too, but surely there is enough space for 2 A319s a week? Just my opinion though.
I have flown between Southampton and Scotland (mostly Edinburgh) over 100 times in the last 7 years, a lot of these flights actually being with Loganair. I noticed on the easyJet flight yesterday that they had attracted a different clientele to what I have seen on Loganair’s flight, so in the long run, I don’t think this will effect Loganair too much.