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.

You probably want to use a working bootloader to see if your ‘hello world’ OS-fragment works at all. No one is writing much OS without running it, surely.

in the bad olden times you definitely started to write your operating system for your new computer from the first instruction executed upwards. it wasn't at all common to have a comprehensive virtualization/simulation environment.