The replacement to segmented stacks is copying stacks, which as far as my knowledge of LLVM takes me, would be very difficult to add. You need a stack map of pointers to successfully move pointed-to objects on the stack from the old region to the new.

There is a great deal of work going on in LLVM on this issue for precise GC of other languages, and (from the outside) it looks like more hours have been spent on it than on the entire Go toolchain. As Go developers don't have the resources or expertise to make such wide-ranging changes to LLVM, it would have blocked Go development.

GCC is similar. Those working on gccgo are trying to work out how to add precise GC and copying stacks. It is much more complex than it was on the gc toolchain.

There is great value in having a simple toolchain that is completely understood by the developers working on it. In fact, that very idea, that code you depend on should be readable and widely understandable, is one of the goals of Go. Applying the goal to the toolchain is a case of eating our own ideological dogfood.