Isn't moving to a new instruction set only a problem with proprietary software? Open source operating systems seem to be able to be ported to new architectures

Isn't moving to a new instruction set only a problem with proprietary software? Open source operating systems seem to be able to be ported to new architectures almost effortlessly, some of them quite old or obscure. It's true that some programs require modification to be able to run on new platforms, but the cost of doing that is minimal compared to the cost of "porting" binary software to new architectures, which is that of simply rewriting it from scratch. Why is everyone talking about emulation and coprocessors when the real issue is basically just a matter of source code availability?