I had this discussion a decade ago and concluded that a reasonable fair scheduler could be built on top of the go runtime scheduler by gating the work presented. The case was be made that the application is the proper, if not only, place to do this. Other than performance, if you encountered a runtime limitation then filing an issue is how the Go community moves forward.
Go's allocator draws from the Hoard work as do most modern alloc/free implementations. Similar C/C++/Rust flavor implementations do not seem to "inevitably leads to memory fragmentation issues". Perhaps this fragmentation concern is a myth carried over from earlier malloc/free or gc algorithms.
I had this discussion a decade ago and concluded that a reasonable fair scheduler could be built on top of the go runtime scheduler by gating the work presented. The case was be made that the application is the proper, if not only, place to do this. Other than performance, if you encountered a runtime limitation then filing an issue is how the Go community moves forward.