Fundamentally, Spectre is a class of bugs that arises from how modern CPUs are designed. It isn't just an Intel bug, an x86 bug, or a JS bug. CPUs that perform speculative execution after a branch prediction are generally expected to be vulnerable. And as I understand it (I'm a security person, not a CPU designer) branch prediction is a very important optimization that is responsible for a lot of the advances in CPU performance over the years. So if CPU designers dropped branch prediction, they'd be trading off a lot of speed for security.