By that logic, any salary < 70% of AWS cost is justifiable.
In reality there is a job market, and there are office politics. These determine salaries more than "marginal revenue of labour". This means that as long as someone who is employable by office-political standards will do the job cheaper, the salary can be contested.
I get your point. But to add to it, there is a labor shortage of good software engineers and that contributes to the high salary. We are still digitizing our economy, we are still disrupting non tech savvy businesses with tech first startups, and we are barely beginning to apply ML all the areas it could make gains. Please see more_corn's comment in this thread about how they haven't found a job yet where they couldn't immediately save the company 10k a month. There is no shortage of work to be done and a real shortage of people who can do it. So the reality is yes, any salary < 70% of AWS cost is justifiable in this example.
In reality there is a job market, and there are office politics. These determine salaries more than "marginal revenue of labour". This means that as long as someone who is employable by office-political standards will do the job cheaper, the salary can be contested.