Julia has GPU compilers for Nvidia, AMD, Intel, and Apple, and we have KernelAbstractions.jl for writing a kernel that is portable between all of them (plus the CPU!)
Just as LLVM doesn't automatically have a backend or every new CPU architecture, Mojo/MLIR doesn't automatically have a backend for every new CPU/GPU/TPU.
However, writing an LLVM backend for RISC-V sure did add support for a whole lot of different programming languages and the software you have access to through them in one fell swoop.
The same is true here.
Instead of rewiting all your GPU code every time you need to target a new GPU/TPU architecture, you just need a new backend.