The x86 tool chains are amazing. They're practically black magic in the kinds of optimizations they can do. Honestly, I think they're a lot of what is keeping Intel competitive in performance. ARM tool chains are also very good. I think they're a lot of the reason behind why ARM can beat RISCV in code space and performance on equivalent class hardware because honestly, like Keller says, they're not all that different for common case software. But frankly x86 and ARM toolchains should dominate RISCV when we just consider the amount of person-hours that have been devoted to these tools.
So for me the real question is, where are the resources that make RISCV toolchains competitive going to come from (and keep in mind x86 and ARM have open source toolchains too)? And, will these optimizations be made available to the public?
If we see significant investment in the toolchains from the likes of Google, Apple and nVidia, or even Intel. ARM needs to be really worried.