git by itself is often unsuitable for XL codebases. Facebook, Google, and many other companies / projects had to augment git to make it suitable or go with a custom solution.
AOSP with 50M LoC uses a manifest-based, depth=1 tool called repo to glue together a repository of repositories. If you’re thinking “why not just use git submodules?”, it’s because git submodules has a rough UX and would require so much wrangling that a custom tool is more favorable.
Meta uses a custom VCS. They recently released sapling: https://sapling-scm.com/docs/introduction/
In general, the philosophy of distributed VCS being better than centralized is actually quite questionable. I want to know what my coworkers are up to and what they’re working on to avoid merge conflicts. DVCS without constant out-of-VCS synchronization causes more merge hell. Git’s default packfile settings are nightmarish — most checkouts should be depth==1, and they should be dynamic only when that file is accessed locally. Deeper integrations of VCS with build systems and file systems can make things even better. I think there’s still tons of room for innovation in the VCS space. The domain naturally opposes change because people don’t want to break their core workflows.