I'm not sure that there's much undue complexity, at least on the kernel side. You just need to ensure that the process running with 32-bit pointers can avoid having to deal with addresses outside the bottom 32-bit address space. That looks potentially doable. You need to do this anyway for other restricted virtual address spaces that arise as a result of memory paging schemes, such as 48-bit on new x86-64 hardware where software may be playing tricks with pointer values and thus be unable to support virtual addresses outside the bottom 48-bit range.
But my main point is that the complexity is not in the one-off "here's a patch to the kernel/compiler to add this", but in the way you now have an entire extra config that needs to be maintained and tested all the way through the software stack by the kernel, toolchain, distros and potentially random other software with inline asm or target specific ifdefs. That's ongoing work for decades for many groups of people.
Defining yet another 32-bit-on-64-bit x86 ABI would be even worse, because now everybody would have to support x32 for the niche users who are still using that, plus your new 32-bit ABI as well.
The academic argument Linus himself made is alone reason enough that big-endian SHOULD be included in the ISA. When you are trying to grasp the fundamentals in class, adding little endian's "partially backward, but partially forward" increases complexity and mistakes without meaningfully increasing knowledge of the course fundamentals.
No zbb support is also a valid use. Very small implementations may want to avoid adding zbb, but still maximize performance. These implementations almost certainly won't be large enough to run Linux and wouldn't be Linus' problem anyway.
While I've found myself almost always agreeing with Linus (even on most of his notably controversial rants), he's simply not correct about this one and has no reason to go past the polite, but firm "Linux has no plans to support a second endianness on RISC-V".
Architectures with only one supported endianness are less painful. "Supports both and both are widely used" would also be OK (I think mips was here for a while?) but I think that has a tendency to collapse into "one is popular and the other is niche" over time.
Relatedly, "x32" style "32 bit pointers on a 64 bit architecture" ABIs are not difficult to define but they also add a lot of extra complexity in the software stack for something niche. And they demonstrate how hard it is to get rid of something once it's nominally supported: x32 is still in Linux because last time they tried to dump it a handful of people said they still used it. Luckily the Arm ILP32 handling never got accepted upstream in the first place, or it would probably also still be there sucking up maintenance effort for almost no users.