FLAC has a decoder. WavPack has a decoder. MP3 (though not lossless) is defined in terms of what a decoder should do, isn't it?
So having to specify decoding is not an argument per se, but of course too high complexity could become troublesome.
DSP instructions, on the other hand, is something that playback software could choose not support, and could end up being "ignored" the same way that those ID3 tags are ignored. I see that issue.