RISC-V is massively simpler (and smaller silicon and lower energy use) for the same performance. Oh, and significantly smaller code size too, so everything from ROMs to RAM (if code is loaded to RAM) to icache can be sized smaller.
The ISA being open source means that if SiFive goes out of business or changes its product lineup, or starts to make buggy stuff, or charge too much ... no matter what happens, the customer is free to find another vendor to make software-compatible chips.
As we've found out with ARM this week, even if one company with ARM's highest and most expensive form of license, the Architecture License Agreement (ALA), makes a CPU design they are not allowed to transfer or sell that design to another company that also has an ALA without ARM's explicit permission. Or so ARM claims. Let's see what the courts decide about that.
Yes you can look at the lower-end cores from both ISAs and compare.
Higher end riscv will come eventually though, it'll take time just like it did for ARM
There's e.g. the SiFive P650[0], from 2021. SiFive does claim it does outperform Cortex-A77.
Cortex-A77 isn't ARM's fastest anymore, but it was in 2019. Meaning SiFive was, in 2021, not even 2 years behind. If tracing back to prior generations, you'll see SiFive has been quickly closing the gap.
To the point that it wouldn't surprise me if at the next iteration from both companies, SiFive and ARM highest performance cores were on par.
What's even more amazing is that SiFive's cores are consistently smaller area and lower power to the ARM cores they have comparable performance to.
I think your question makes a lot of sense in the general case, but in case of NASA it very well might be that they scratched their heads, came up with requirements and found that they don't care about there being an ecosystem of any kind due to special nature of their requirements.
The advantage is not NASA's, but SiFive's, or any other company that wants to make their own chip: they don't have to pay a license. I guess it is NASA's advantage too, because their software shouldn't be tied up with any limited group of license holders.
Open ISA is huge. It may not mean open hardware on its own, but it sure as hell opens the door to open hardware in the future. I think there is a chinese CPU in development that is claiming to aim to be the linux of CPUs. Can't remember the name but they were at one of the risc-v conferences recently.
