Git's biggest flaw is that it doesn't scale. If a new system can fix that without sacrificing any of Git's benefits, I think it can topple Git.
It's ironic that Git was popularized in the same era as monorepos, yet Git is a poor fit for monorepos. There have been some attempts to work around this. Google's `repo` command is a wrapper around Git that treats a set of smaller repos like one big one, but it's a (very) leaky abstraction. Microsoft's GVFS is a promising attempt to truly scale Git to giant repos, but it's developed as an addon rather than a core part of Git, and so far it only works on Windows (with macOS support in development). GVFS arguably has the potential to become an ubiquitous part of the Git experience, someday... but it probably won't.
Git also has trouble with large files. The situation is better these days, as most people have seemingly standardized on git-lfs (over its older competitor git-annex), and it works pretty well. Nevertheless, it feels like a hack that "large" files have to be managed using a completely different system from normal files, one which (again) is not a core part of Git.
There exist version control systems that do scale well to large repos and large files, but all the ones I've heard of have other disadvantages compared to Git. For example, they're not decentralized, or they're not as lightning-fast as Git is in smaller repos, or they're harder to use. That's why I think there's room for a future competitor!
(Fossil is not that competitor. From what I've heard, it neither scales well nor matches Git in performance for small repos, unfortunately.)
It's ironic that Git was popularized in the same era as monorepos, yet Git is a poor fit for monorepos. There have been some attempts to work around this. Google's `repo` command is a wrapper around Git that treats a set of smaller repos like one big one, but it's a (very) leaky abstraction. Microsoft's GVFS is a promising attempt to truly scale Git to giant repos, but it's developed as an addon rather than a core part of Git, and so far it only works on Windows (with macOS support in development). GVFS arguably has the potential to become an ubiquitous part of the Git experience, someday... but it probably won't.
Git also has trouble with large files. The situation is better these days, as most people have seemingly standardized on git-lfs (over its older competitor git-annex), and it works pretty well. Nevertheless, it feels like a hack that "large" files have to be managed using a completely different system from normal files, one which (again) is not a core part of Git.
There exist version control systems that do scale well to large repos and large files, but all the ones I've heard of have other disadvantages compared to Git. For example, they're not decentralized, or they're not as lightning-fast as Git is in smaller repos, or they're harder to use. That's why I think there's room for a future competitor!
(Fossil is not that competitor. From what I've heard, it neither scales well nor matches Git in performance for small repos, unfortunately.)