Comment by nostrademons

I assume you mean "11000000 10000001" to preserve the property that all continuation bytes start with "10"? [Edit: looks like you edited that in]. Without that property, UTF-8 loses self-synchronicity, the property that given a truncated UTF-8 stream, you can always find the codepoint boundaries, and will lose at most codepoint worth rather than having the whole stream be garbled.

In theory you could do it that way, but it comes at the cost of decoder performance. With UTF-8, you can reassemble a codepoint from a stream using only fast bitwise operations (&, |, and <<). If you declared that you had to subtract the legal codepoints represented by shorter sequences, you'd have to introduce additional arithmetic operations in encoding and decoding.