Concurrency is not a big problem. The only concurrency issue with Git is that refs (and states like rebase/merge conflicts) are stored in "loose" files. This

Concurrency is not a big problem. The only concurrency issue with Git is that refs (and states like rebase/merge conflicts) are stored in "loose" files. This can be easily and elegantly solved like how jj does it, by putting the repo metadata into the object store too. You can use a jj repo concurrently, e.g., over Dropbox with coworkers, and all it requires is a minor modification on top of the existing Git data model.