> What's really need though is a universal, stable over eons, single standard for time, length, and mass. I believe time is N cycles of an excited sodium (light) emission. Length is N wavelengths of that same emission in a vacuum. Mass would be N atoms.

Turns out they know that :-)

The problem isn't definition, its the accuracy of reproducibility. You need a mechanism that can be built from scratch and can be believed to produce identical results. You can pull that off counting wavelengths of an unknown number of a known pure atom. Counting actual numbers of atoms is quite difficult. Typically we do it statistically...via mass! We could get it precisely by using a small number, but then actually measuring the mass (rather than calculating it) would be difficult.

A second (time) is already defined on cesium transition, a meter (length) is defined as a fraction of the distance light travels in a second. Both things which we can accurately measure for some time now, and which universal and stable over eons.

What is the problem that would be solved by switching to a single element?

I don't recall. I just read some article recently about trying to define time and length with sodium; trying to define everything with a single element that is very common. Of course it could also have been a 50 year old Asimov book.

They reference that silicon definition later on in the article.