Memory-safety means it doesn't matter whether you're running in a separate address space or not; you're equally isolated either way. To isolate two components, all you need to do is... not give them a reference to each other. Memory-safety means no matter what they do, they can't interfere with each other's memory.
Of course, operating system components might be able to interfere with each other through, perhaps, access to privileged instructions or access to hardware; but in a memory-safe language, constraining access to privileged instructions or hardware is just a matter of putting it behind a suitably safe interface. It doesn't matter if that interface is implemented with direct function calls or with cross-address-space RPCs, they're both equally safe.
A microkernel (and its RPC framework) is ultimately a poor implementation of a module system; that is, a system for breaking up a program into components and then allowing them to communicate. But any modern programming language already has a high-quality module system, and those modules can communicate through the most powerful and efficient mechanism: Direct function calls. In the old days, with memory-unsafe C, language-based modularity wasn't safe, and microkernel-style address space isolation was our only option. But we have much better options today, with memory-safe languages.
One old argument in favor of microkernels was that they would allow for more customizability and modularity; that it was impossible for a monolithic kernel to be modular. These days, we have Linux, which is an extremely modular kernel, with thousands of modules which can be combined in many different arrangements. Far more modular, in practice, than any microkernel has ever been.
The only remaining "benefit" of a microkernel's approach to modularity is that you can distribute a microkernel module as a binary, without source code; a memory-safe language, on the other hand, requires access to the source code so that it can compile it to safe binary code. But, fortunately, we have a robust open source operating system already - Linux - so we don't need to make any compromises: We can reasonably require that all (significant) modules be distributed as open source, because that's in fact what Linux already requires.
There's no advantage to the microkernel architecture, if you're writing your operating system in a memory-safe language. I found this quite surprising when I first heard it, but when I finally understood it, I also understood that it points the way towards far more advanced and powerful operating system designs. The microkernel architecture is a relic of the memory-unsafe past; the future is memory safety, and we should take full advantage of that.
(This article is deeply indebted to Faré's article on microkernels, which is where I first heard this idea. That article, unfortunately, is dense, difficult to understand and filled with unrelated ideas, so I'm writing this article to repeat just this idea, hopefully more clearly.)