Only supporting UTF-8 means their 8-bit text files will not be readable. It also means you can't use grep on binary files anymore. The meaning of Unix streams is ambiguous between bytes and text, with binaries often flowing through pipes.
But in general, there's just this sense of "I don't understand why they made these features, so they're wrong", "My preference is like this, so I will present it as the one correct way", "I need a bignum library so I'm going to roll my own even though there are great, mature, focused libs out there for it already", etc. It all smells of hubris, and there's little good reason for it to be such an isolated NIH project.