> if you went ahead and extended the standard to simply allow more bytes, you could get up to 8 bytes

I think you'd only be able to go up to 7, since 10xxxxxx is still reserved for trailing octets. And even with 7, the entire first octet is consumed by the length indicator alone.

So you get 0xxxxxxx, 110xxxxx, 1110xxxx, 11110xxx, 111110xx, 1111110x, and 11111110 as the 7 different length-indicating head octets. In the last case, you'd have 36 usable bits for encoding a codepoint.