I worked on a team that built a massive, high-performance internal service based on Riak. There are many things I learned from that system. Here is the best takeaway I can offer:

It does not matter what your technology is, or how theoretically superior it is. Getting it to actually work well "in production" is a whole separate thing than simply designing it and writing code. When it's a very small system, it will look like it's doing great. As it gets bigger, the seams will start to burst, and you will find out that promises and theory don't always match reality.

In the end, while its aims are great, it takes a whoooooole lot of work to smooth out the bumps in such a system. You need experts in that technology to address bugs in a timely manner. You need developers versed in the system to properly build apps utilizing it. You need competent operators to build, orchestrate, operate and maintain the whole thing.

All of that is made easier by using simple technology that everybody knows, that there's a huge support community for, professional services for, etc. A technology like MySQL or Postgres etc, has the corporate, development, support, etc to make it easy to work with at any scale. A little janky at times, limited, but dependable, predictable, controllable.

A small bespoke system with a small support community and virtually no corporate support is, comparatively, a hell of a lot more difficult/costly to support and harder to make work reliably.