Software can choose to invalidate all cache, fence access, and the like today. It may not be a single instruction but it's not far off. Usually something like "just don't JIT JS to native code" is "secure enough" most don't want to go down that route. The mitigations are the way to keep the most overall performance, so they are the most popular.
Of course, it’s not doing much for the billions of devices that exist.
I would hope that we could find a software solution that web browsers can implement so that devices can be patched.
Either way, I would want such a solution to not compromise performance in the case where code is running in the same security context.
This is what I don’t like about existing mitigations. It’s making computers slower in many contexts where they don’t need to be.