Interesting point, and I'm included to agree with your main point. I don't think the OS level is preferable, however:

Point 1: I do not want my program to only run on only one OS, or to require custom code to make it multi-platform.

Point 2: What if there's no OS?

>Point 1: I do not want my program to only run on only one OS, or to require custom code to make it multi-platform.

To run on only one OS at build time? I usually just set up cross-compilers from linux if I am making cross-platform C/C++ code.

>Point 2: What if there's no OS?

You can use a system like bitbake I think.