In fairness I don't think it's quite as much of a no brainer as you'd think. Firstly, when RISC-V was developed Rust was still pre-1.0. People didn't think it would amount to anything. So most high performance code was C/C++ which doesn't have checked arithmetic.

Second, it's easy to say "trap on overflow" but traps are super annoying. You really ideally would want to avoid leaving user mode. As soon as you trap to the OS you're now dealing with signals which are pretty much the worst thing in the world. The 4 instruction case at least lets you just branch to other code.

So you ideally want an "add or branch" instruction, but there isn't enough space in the opcodes for that. The fallback is flags, which also massively suck. I don't know if anyone has a great solution to this problem.