Human angle in Software development

No matter what methods(Agile, traditional, Evo, FDD, RUP, etc) that is applied for software development, the only thing that decides the fate of the project is people.

There are many traditional projects that have been delivered successfully, and Agile projects that have failed miserably. The success or failure of projects doesn't depend on the process or framework that is being used.
According to me, it all depends on the people who are planning ,executing and maintaining the project. When I say people, it need not be the poor software developers, it could be PMs, customers themselves, marketing/sales people, finance team, etc.

The project's fate gets decided right from the time the marketing/sales person starts selling the skills/services to the prospective customer. I have seen many instances where the marketing/sales team over commit with the customers for the sake of winning the project. Ultimately when the actual work reaches the architects/project managers/developers, there will be little room left to make any changes. In most of the projects, the timeline is fixed, the only thing that is allowed to be flexible would be resources(a few lucky ones).

Even after so much of campaign happening around software process improvement, the project stakeholders add/remove people to the projects based on simple arithmetic. They don't look at software developers as humans but instead like chairs/tables/"things" that can be replaced and used right from the time it is purchased.

Here is an example and explanation of the above paragraph:

Let us say it takes 400 person days worth of effort to implement a feature . Naturally the stakeholders will calculate and say that if 20 people work on this feature for the next 20 days this work could be completed.

There is no flaw in the above calculation. However, the stakeholders never even think of buffers due to attrition or addition. What I am trying to say is, let us say the above team of 20 people start working on the project and they have completed 10 days (i.e. 20 P * 10 D = 300 PD complete). Let us say, the 11th day a team member quits the project. Consider the best case scenario where they found a new replacement person the same day.

My question is,

Even after finding the replacement the same day, what are the chances of completing the project as scheduled within the remaining next 10 days ?

According to me, the delay would be more than 5 days. Reason being, when a new person is introduced into any project, it not only takes some effort to psychologically adjust to new team but also takes effort in understanding the work. Unfortunately, in most of the cases I have seen the stakeholders don't consider the human angle and still push people to complete the work as planned because the new replacement was found the same day !!. This push in turn leads to reduced thinking due to time line pressure and ultimately reducing the quality of the work.

In Slack, Tom Demarco puts this process of replacement as "personnel turnover". As per research, the effort and the cost associated with personnel turnover could be as big as 25% of overall project effort. The cost may not be visible directly but it gets incurred due to delivery delay, training new personnel and defects fixing.

Until human angle is not given at most importance in any project, the project will go through turmoil no matter what method they follow !!