People as single point of failure

Answer:

Your question is more focused on business continuity, so I'll describe how is this usually resolved from ISO 22301 perspective - there are a couple of strategies on how to approach this problem:
a) Employ someone else who can replace certain people that are crucial
b) Write very detailed procedures so that other employees can resume doing the job
c) Make a contract with an external party that can provide you with human resources with particular skills in case that your crucial people are not available.

See also this article: Can business continuity strategy save your money? https://advisera.com/27001academy/blog/2010/03/15/can-business-continuity-strategy-save-your-money/