Office 365 is was introduced in 2010. So when it was created Microsoft had two reasonable options: 1. Accept the performance penalty from using a safe garbage

Office 365 is was introduced in 2010. So when it was created Microsoft had two reasonable options: 1. Accept the performance penalty from using a safe garbage collected language (such as their own C#) which is slower which may make the service less attractive and more resource hungry which means more expensive to run at scale. 2. Accept the correctness penalty from C++ which delivers good performance but will cause an endless stream of bugs, some very hard to diagnose. Rust makes this much easier, because you aren't paying the correctness penalty, your Rust will probably have the same or fewer correctness bugs as your C#, and yet you get the good performance you'd have wanted from C++. For a 2010 product Rust was not an option, but presumably at some point in the last say five years, Microsoft did the maths and worked out that unless they're killing Office 365 soon or somehow C++ magically becomes safe with no work, a transition to Rust is cost effective.