Earlier quoted context omitted.
You're leaving out some serious costs incurred by your company and team. The loss of institutional knowledge, the loss of expertise, business process knowledge, personal relationships that could make hard changes much easier. Sure, you hired someone for $X instead of $X*1.2 but in terms of work done you probably missed out on a lot and came out way behind.
I think you're both overestimating those costs and also underestimating the difficulty of just being able to suddenly pay 1.2X for people. Re: institutional knowledge and expertise, etc - we tried to avoid too much concentration. If all the institutional knowledge for system X is tied up in person Z, what if they want to take on to a new roll, or I want to ask them to work on a different system or a new project? I li…
It's like trying to argue that a mechanic who has only worked on Ford vehicles for 20 years should be able to quickly have the same proficiency as a mechanic who has 20 years of experience on only Porches. Nonsense.
Remember, a company can typically expect to hold onto a developer for maybe 3-5 years, so even half a year of training is a significant portion of their tenure.