Binary size is a small percentage of the overall asset bundle.
Docker also supports multiarch images.
Given how easy it is to JIT or BT RISC ISAs, the future is good for binary portability.
I’m thinking of all of the small utilities and small command line programs that make up a stock Linux distro. Those don’t have many resources other than the binaries. Sure, the size of each is not much in absolute scale, but combined, you have a pretty significant increase if they were all fat binaries.
That said, I don’t know what Apple does. For example, in the main download for Big Sur, is (for example) zsh a universal binary, or are there a specific x86/M1 downloads. I haven’t looked.
Looking at /usr/bin, /usr/lib/ etc. it's not quite as big as I expected. Going fat might be feasible, especially considering how large storage is at this point.
Universal binaries. Apple calls those binaries “Universal binaries”. Rosetta is more like running different arch binaries through qemu. A brief look through Google says that there was a FatELF specification created years back, but never really went anywhere. Presumably because Linux users tend to know what arch they are using.
Fat binaries would make distribution easier, but would double (or triple) the size of a binary. I doubt it would be worth the size trade off.