I think Ubuntu (and other's desire) to remove coreutils is precisely because coreutils is old, brittle and written in an unsafe language. So they've chosen to modernize the code base, attract new developers while still maintaining strict compatibility. And since uutils is MIT instead of GPLv3, and far more portable between operating systems it may encourage adoption elsewhere, e.g. Microsoft already have build for example.
That said, rewriting code always carries risk and uutils has its own set of bugs which have been uncovered and need fixing. There is a "if it isn't broke don't fix it" mantra but I suppose it depends on the definition of broke - if development is moribund then that could be considered an issue too.
It's worth noting that coreutils have derived benefit from the exercise too. The extensive test suite for uutils has uncovered bugs in coreutils and improved the overall code coverage. So even if people don't want to switch (even though they're functionally interchangeable), the existing package has derived benefit too.