The thing is that this is not actually going to happen. Programs are simply broken across the board, because few people can be bothered to deal with all these peculiarities.
The difference is, in Python 2, output would be corrupted in some edge cases, but generally it would "just work". In Python 3, the program falls flat on its face even in cases that would've ended up working fine in Python 2.
I don't think there's a general answer on which behavior causes less real-world problems total, but the idea that Python 3 makes less of a mess is not something I can agree with.
Excuse me, I don't exclusively deal in 7-bit ASCII characters just because I happen to speak English, which isn't the only language I speak either.
It's just the way it is.
Python 3 completely removed the need for us to talk about unicode and encodings in our third semester data analysis workshop for physic students because it just works with umlauts and Greek letters. Python 2 was a real pain.