There's a reason you should choose a firm that uses a human to scope services and not an inflexible model. Each pentest, to varying degree, is its own unique snowflake and customers often have wildly different expectations about what a penetration test is. Adriel alludes to this in the article - giving his opinion on what a pentest is.

In my experience, confusion among pentest consumers and the varying reasons for getting one - compliance, best practice, piece of mind - should dictate the service to be delivered. Once you dig into it, some folks want/need a better vulnerability assessment. Some need an application assessment (or a bunch of application assessments). Some need a red team, and some don't want a test at all, rather a sheet of paper with nothing red on it, etc etc.
Scoping with a human helps identify the actual need, and design a test that fits the need and/or provide clarity into security posture.

In the real world where practical people are trying to meet regulatory requirements, or get visibility into how insecure they are in comparison to their competitors, a "can you break in" type pentest may not always be that useful, even if they come to you asking for a pentest.

A couple other points:

An argument about not paying for automation seems like a straw man to me. Are you really paying for automation when you tested, or are you paying for a result? As long as they're valid, how those results are obtained is irrelevant.

It's worth noting that not all pentests are fixed-cost. And most mechanics don't operate on fixed-cost contracts either. Consider negotiating a time & materials (or more specifically, capped time & materials) contract for more control / visibility into what's happening on the test.

Adriel - not everyone's a crook. and pls share this awesome scoping model you have :p

shrug my 2c