@p I'd like to point out that, GNU had in the past put a lot of work into ensuring that existing free software on Unix could be compiled on GNU without having to be re-written and of course such compatibility remains.

Such compatibility was implemented long before Linux was even a thing.

Linux does barely anything really - GNU bash passes the elf to glibc's ld-linux-x86-64.so (or ld-hurdxxx.so), which loads the program for execution and all Linux really does is schedule the machines resources (the program is allowed unfettered access to CPU cycles until its scheduled time is over and then Linux goes and halts execution and dumps the current execution state to a stack and runs the scheduled program) and implement SYSCALLs (via glibc, which does a lot of things internally, but does need to call SYSCALLs for things like a read or a write to a file).

Although efficient scheduling and SYSCALLs and drivers are exiting as they are complicated, they are a far cry short from what you need to get an operating system.