Considering Apple have custom extensions to ARM already, they'd be giving up more than just the SoC design if they switched to a standard ARM core, unless eventually all those extensions get upstreamed into the spec... but I don't see GXF [1] getting upstreamed, at least not like that. That one is just crazy.

[1] https://blog.svenpeter.dev/posts/m1_sprr_gxf/