I don't understand, how do these low level changes impact the OS exactly assuming that the ISA remains the same? It doesn't seem much more impactful than SSE/AV

I don't understand, how do these low level changes impact the OS exactly assuming that the ISA remains the same? It doesn't seem much more impactful than SSE/AVX and friends, i.e. code not specifically optimized for these features won't benefit but it'll still work and people can incrementally recompile/reoptimize for the new features. After all that's pretty much how Intel has operated all the way since the 8086. It's not like Itanium where everything had to be redone from scratch basically.