Unless the customer is one who has issues with large downloads. Not everyone has fiber to the home with massive storage on modern computers. Some people have the entry level systems, settle for slow internet. Often they are in poor or "third world" places which means maybe you don't care about them, but they exist.

Given how fat modern websites are, compassion for the bandwidth deprived seems rare.

Very much. Which is really a problem for those who do have bandwidth limitations. Or even low end phones / old computers. Or even new computers that are not very powerful.

Fat binaries are fine. Electron is a fat binary in a different sense.

I don't think macos will allow you to run a downloaded cosmo binary without going into security settings and enabling it. That's not an experience I'd want my customers to have personally which means if you care about mac normies, this isn't a viable approach.

I don't know what magic is involved, but I was able to download eg https://cosmo.zip/pub/cosmos/bin/md5sum and run it in a terminal without having to do run 'xattr -d com.apple.quarantine' on it.