Good question, I should probably have clarified that better. I get more into that in my comparison of Carbon and Swift:

https://itnext.io/google-carbon-vs-apple-swift-9df8262342c8

The reason I compare Swift and Carbon is that going from Objective-C to Swift we faced exactly the same problem as you point out: What do you do if you have a large code base in Objective-C. How will you migrate all that code to Swift?

Basically you don't have to. When you got a drop-in replacement, you can easily deal with having both languages in the code based an migrate over time.

It works very well when you can call code in both direction. C++ code can call Carbon code, and Carbon code can call C++ code. We had the same benefit with Swift which made migration extremely smooth.

If they can replacate what Apple did with Swift for C++, then Google will have a great solution for C++ developers.