if all text is UTF-8, then most operations can be performed on bytes without worrying about any transformations whatsoever
This is incorrect for anything other than an ASCII-only world. In general, "just use UTF-8" is a shorthand for "just let me pretend non-ASCII doesn't exist", because it leads to people writing code that assumes one byte == one character.
By rubbing your nose in the difference between str and bytes, Python 3 complicates your life.
Having had a lot of experience with it, I would instead say Python 3 makes you think about things and find bugs at the appropriate time -- up-front during the development process -- rather than at an inappropriate time, like 3AM on a Sunday when your pager goes off because you trusted the "helpful" but wrong way of doing things.