They did start with C++. They experimented with Swift, then experimented with Rust, ultimately choosing Rust. I don't see how experimenting with things makes anything "not serious". If you don't question your primitives and just accept them "because that's how it has always been", you're not a very good engineer in my book. In any case, the tech stack is not in constant flux as you claim, it's still C++, with more and more parts moving / being started in Rust, with the eventual probability of Rust replacing C++ organically, I suppose. Seems like pretty sound strategy to me to slowly, but surely, adopt a memory-safe language.