>> Will this really make coreutils more secure?

In the long run yes. Sure, many logic bugs happen in Rust programs as well, but memory safety bugs are another level of hell. At least many classes of exploits will mostly be impossible. Also, Rust program failures tend to be more predictable. For example, in C/C++, if you do out of bound writes in an array or writes in a freed memory block, the behavior is undefined. The program might crash, the memory might be silently corrupted, or nothing might happen at all. In a normal (non unsafe) Rust code these kinds of issues are either prevented by static checks or become explicit runtime panics.

While the migration could be done more gradually, 26.10 is not an LTS release and is not considered stabe enough. The next LTS is 28.04, so I hope there is 1.5+ years for things to stabilize.