Depends on the purpose of making the language. Carbon really aims to replace C++ by targeting not only compatibility, but also interoperability, much like Objective-C approach to C. Thus, every valid C++ code is a valid Carbon code. This has been somewhat proven in the past to boost psychological confidence that helps adoption. I don't know what Google has in mind, but probably this approach can be used as a lure first, eventually they will add "better" replacements for constructs they consider bad, which naturally leads to termination of C++ IF those replacements succeed, i.e. they're used and preferred. At that point, they can drop C++ compatibility/interoperability and Carbon becomes standalone language.
What you stated sounds quite reasonable and, I have to admit that I believe their motivation is as you stated it.
Myself, I see the situation in computer language design as being quite similar to the situation with car manufacturing back in the 70's. Back then, the U.S industry was producing low quality cars (pretty close to junk) and selling them for as much as they could get (as economists say, "charge what the market will bear"), in the meantime, the Japanese were were focused on making an ever better car. At the end of the day, the better car won the day and the U.S industry is still trying to figure out how to build a car as good and reliable as a Japanese car.
I view compatibility with C and C++ as an attempt to copy a rather _deficient_ design. (that said, I do think that the C language has a reasonably good feature set, just very poorly implemented.) There are a lot improvements that could be part of a new design but, compiler writers seem to be stuck putting more lipstick on a pig. The only reason that may end up working is because the result may end up being "the least bad alternative" instead of something genuinely good.
Another thing I find interesting, is that, there are _countless_ books on how to write a compiler but, very _few_ (personally, I haven't even seen one) books about how to design a programming language.