Hiring a COO is justified, for a time.
So when does the role become excessive overhead?
When change momentum slows. Fresh ideas and skills eventually lose their impact. When the job is done. Once the groundwork is set, hiring in-house becomes more cost-effective. When the company evolves. If the new reality no longer fits the COO's skill set, does the role still add enough value?
This is why a COO is inherently an interim role. One that, when done right, reduces overhead.