Just because the ski season is over, and there is more demand for beach holidays, doesn't mean BOH-GVA can't work year round. Might be worth looking in a summer 'Lakes and Mountains' holiday brochure? And that would just be one potential source of summer passengers. Second home owners, and maybe even limited business traffic would be attracted by a year round service
What maybe the issue is are there better uses for the aircraft in the summer, such as flights to 'beach' destinations?