Is there any important particular reason why the kernel should always
guarentee to provide more memory to a demanding process (or force it
to sleep until it gets some), rather than immediately reply that the
request could not be fulfilled? (memory and swap temporarily full)?

It would then consist of the process's responsibility (as well as libc)
to ensure to properly check error conditions and act appropriately,
the kernel would never find itself in any unpredictable state... Of
course badly written software assuming that memory is always available
would segfault as expected trying to access unallocated memory...
libc standards always define malloc() error conditions, which it could
report to the user program when the kernel failed to provide the
requested resource immediately, which a program should take into
account

Thanks
Matt