Depending on what that business software is doing I wouldn't be surprised in a few hundred thousand lines were written in something else. Microsoft (https://medium.com/@tinocaer/how-microsoft-is-adopting-rust-...), etc, are all doing this right now. But yea if you are writing low velocity, non mission critical software, no need to bother. On the other hand, there may come a point in time where that c++ project gets a boost by factoring the critical parts out of it into a safe language...
The one I’m working on is a behemoth and already has several hundred thousand lines of other languages (python, Java, scala) as part of it.
The c++ will be staying c++.
I don’t say this out of any affinity for c++(check my other comments for details), rather because for business reasons, the core of this system will always be c++.
In fact we a currently embarking on a multi-million dollar project to upgrade the application to run in the cloud and one of our biggest challenges is finding qualified c++ candidates.
Despite that, due to the complexity and scope of the application and years of business knowledge captured in obscure parts of the code, there is no consideration for it to be rewritten in anything else.
P.S. the software is absolutely mission critical (responsible for a non-trivial % of total company profit), hence the reason it's not going to get rewritten without a very, very compelling business case.
For example, the current c++ project I work on (for a large company) has over 1.5 million lines of c++, spanning over 5,000 files, with business knowledge and bug fixes spanning over 30 years.
It's not going to get rewritten in anything else, and the same goes for large amounts of other business software.