I think the answer is dependent on how good the coach is. If the coach is well-liked then stability is good bc kids look forward to swimming for the coach season after season.
But if the coach isn't great, no reason to keep them to preserve longevity.
As for how hard to find new coach, probably depends on what your pool can offer in terms of salary and what is required for the job at your particular pool.
I don't mean to not answer your question but I really don't think there is a universal answer.