* this article acknowledes that while open-source hardware requires an open-source ISA, this requirement is not sufficient
* this article mentions open-source hardware that implements the open-source RISC-V ISA, and the type of applications and research that this hardware is used for (e.g. eliminating Spectre-like timing attacks). It also mentions how such research is pretty much impossible to publish for non-open-source hardware using non-open-source ISAs.
Yes, That's the main point IMHO. It's firstly about making research easier and decoupled from some companies, allowing you to then push given companies to adopt research. It also slightly increases the chances for more competition on the CPU marked by making entry easier for new companies. Lastly it's useful for countries which want to become less dependent on the US.
The idea that somehow an open source ISA will lead to open source hardware which will replace well established hardware is a bit to optimistic in my opinion. But it can do a lot good even without it.