What you say works 100% for trivial CRUD applications. Which is also, where SCRUM is still a bad framework, but at least SCRUM works on this trivial level of software development.
When a project is about non trivial projects, people, especially developers can not be easily replaced, because no one can replace a specialist with several years of experience on the spot. What I can archive in one day and what somebody else can archive on a day highly depends on what it is. Write a field to the database? Works. Write a rule engine? I'll probably be a factor of 10x more productive than an average developer. Write some GUI/CSS? A frontend developer is 10x more productive than me.
When a project is about non trivial projects, people, especially developers can not be easily replaced, because no one can replace a specialist with several years of experience on the spot. What I can archive in one day and what somebody else can archive on a day highly depends on what it is. Write a field to the database? Works. Write a rule engine? I'll probably be a factor of 10x more productive than an average developer. Write some GUI/CSS? A frontend developer is 10x more productive than me.