Check out this post by Rand Fishkin of SEOMoz fame on the topic, where he advocates for hiring consultancies https://sparktoro.com/blog/why-you-should-hire-agencies-cons...

To be fair, I think your point about "except for very boutique consultancies" may apply to his recommendation. I can't imagine him advocating for an Accenture or similar, instead the message is bring on expertise for specific initiatives as needed.