"Linux is still a hobby OS. It's not a particularly wonderful example of software engineering. Its only real benefit is that it's free and comes with an ecosystem of other free software that kind of mostly works as long as you don't mind the occasional security horror, and can be used as-is or customised at relatively low cost."
That is complete nonsense. Linux, particularly Red Hat Enterprise Linux is as serious of a server OS as exists in the world today. Companies like IBM and Oracle would never embrace a "hobby" OS in an enterprise setting.
As for "occasional security horrors", sadly there is no OS of any flavor that is immune.
"So what business problem does RISC-V solve?"
For one thing, RISC-V offers the promise of fully open computer systems, without opaque black boxes anywhere providing potential back doors or other problems. The Intel AMT vulnerability is an excellent example of how that can go very wrong:
RISC-V also provides a playground for smaller entities (like university labs) wishing to experiment with innovative new hardware techniques like Unums. That's very valuable in its own right.
Unums are unlikely to gain much usage. Posits, also by Gustafson, are a more reasonable alternative to IEEE-754 floating point (but will still have a difficult time displacing IEEE-754, if they can at all).
You need to be a lot more explicit about what you’re asking. Look at the slides, searching down for “At nbits = 5, fraction bits appear”. Notice that every possible bit pattern is used and meaningful.