Except you can write bootloader only if you have something to boot.

Bootloaders as we know them came after OSes, because the need of a bootloader is to boot something. Following your logic of building a piece only if needed, a bootloader should come after.

I like the recipe analogy and I actually think adding things only if needed will help cutting unuseful features and possibly to have different implementations for solutions you need.

No point in reinvent the whole thing, tho. Following your analogy, if all cakes need egg, there must be a reason even if we don't know it.