It appears that the problem is the fall in the GBP which would be likely to affect all UK airlines to a greater or lesser degree. It seems the underlying model is robust.
Stelios has been a vocal critic of the airline for a long time and his fears for the airline a few years ago never materialised in actuality. He might be right this time or he might not be - only time will tell.
If easyJet did hit seriously choppy waters causing it to downsize significantly it's probable that it would not be affected in isolation because the market generally would likely to be the cause.
As for the still fewer flights in peak summer compared with spring/early summer, it might be thought that the spring/early summer period would have been the one to have seen the reduction as fares, loads (and presumably yields) are higher in July and August.
Relying on one customer is never ideal for any business and BRS does lean heavily on easyJet (about 50% of its passenger numbers) but then again, as I pointed out a few weeks ago, so do a lot of other smaller airports.
BRS has been there before. In 1974 passenger numbers declined by 36% on 1973 when Court Line (the airport's major holiday carrier then) ceased to trade. Passenger numbers then were tiny (under 300,000 a year) but the effect on the airport was still substantial.