In future it might be more reliable and faster, maybe with more features.

But we probably won't see any effect for 10 years or so.

Except there are far less Rust developers than C developers, so contributions will start to drop as Rust usage expands in git.

I would safely bet that the pool of C developers willing to work on a C Git going forward is much closer to exhaustion than the pool of Rust developers willing to work on a Rust(-ish) Git.

10 years? are they going to contribute 1 line of a code a day or something?

Well it would probably take at least 5 years to rewrite all of Git in Rust (git-oxide is 5 years old and far from finished). Then another few years to see novel features, then a year or two to actually get the release.

Btw 10 lines of code per day is a typical velocity for full time work, given it's volunteers 1 line per day might not be as crazy as you think.