Uppercasing/lowercasing cannot be done on Unicode code points, because that fails to handle things like fi -> FI where the uppercased version does not consist of the same number of Unicode code points. Slicing and splitting cannot be done on Unicode code points because it may separate a character from a subsequent combining character. "startswith" cannot be done on Unicode code points because some distinct code points need to be treated as equivalent. These are pretty much the same problems you also have when you perform those same operations on bytes. You might encounter those problems in fewer cases when you perform operations on code points rather than on bytes, but you won't have solved the problems entirely.
Worse, you'll have pushed the problematic cases out of the realm of obviously wrong and not sensible to do, into subtly wrong and will break down the line in ways that will be hard to recognize and debug.
