I'm used to people arguing for simpler setups because the belief is that they could make them more performant. This was specifically the push for RISC back in the day, no?

To that end, I was assuming the idea would be that we think we could have faster systems if we didn't have this stuff. If that is not the assumption, I'm curious what the appeal is?

That’s certainly not the assumption here. The appeal is, as I said, that the systems would be more predictable and tractable, instead of being a tarpit of complexity. It would be easier to reason about them, and about their runtime characteristics. Side-channel attacks wouldn’t be a thing, or at least not as much. Nowadays it’s rather difficult to reason about the runtime characteristics of code on modern CPUs, about what exactly will be going on behind the scenes. More often than not, you have to resort to testing how specific scenarios will behave, rather than being able to predict the general case.