The trick is getting everyone to switch over and ensure correct security and correctness for the newer software. A good example may be openssh. It is very well established, so many will use it - but it has had some issues over the years, and due to that, it is actually _very_ difficult now to know what the _correct_ way to configure it for the best, modern, performant, and _secure_ operation. There are hundreds of different options for it, almost all of them existing for 'legacy reasons' (in other words no one should ever use in any circumstance that requires any security).
Then along comes things like mosh or dropbear, which seem like they _may_ improve security, but still basically do the same thing as openssh, so it is unclear if they have a the same security problems and simply don't get reported due to lower use, or if they aren't vulnerable.
While simultaneously, things like quicssh-rs rewrite the idea but completely differently, such that it is likely far, far more secure (and importantly simpler!), but getting more eyes on it for security is still important.
So effectively, having things like Linux move to Rust (but as the proper foundation rather than some new and untrusted entity) can be great when considering any 'rewrite' of software, not only for removing the cruft that we now know shouldn't be used due to having better solutions (enforce using only best and modern crypto or filesystems, and so on), but also to remodel the software to be more simple, cleaner, concise, and correct.
I don't see why. People will just discover they rewrote something slower.