Recent research across the FTSE 350 in part emphasised the longevity of auditor/client relationships (see https://www.accountancyage.com/aa/news/2197422/ftse-350-use-same-auditor-for-an-average-of-113-years )
What is perhaps more interesting is the reasons which were discovered to be most likely to prompt a change in auditor. These included complacency of the audit firm, a substantial increase in audit fee, or a problematic relationship between auditor and management.
The first and third of these clearly represent risks, and eventual loss of profitable business that could be avoided. Its worth reviewing your most profitable relationships. Are all of them risk free? Are you sure?