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.