People often forget that 10X does not (only) mean quantity, but most of all quality.
Also thinking in factors may not be the right approach here. We should look at it in a more binary way: Can a developer achieve the requirements or not? In most cases the magnitude of skill shows in what a developer can solve at all, not how fast he/she can solve it.
Memory safety issues are bugs. Do you know any programmer that does not occasionally create bugs? Don't forget tight schedules, low budgets, ...
Also rust is just what you propose that - a programming language integrated with heavy verification of safeness built-in. Because occasionally someone writes c code without using all available tools to verify the code it is better to have it built in.
Also thinking in factors may not be the right approach here. We should look at it in a more binary way: Can a developer achieve the requirements or not? In most cases the magnitude of skill shows in what a developer can solve at all, not how fast he/she can solve it.