I'm with you on undesirability of silent change of behavior. But requiring people to make an explicit choice would immediately break a lot more code, because now all the (far more numerous) instances of code that genuinely doesn't care one way or another won't run at all without changes - and note that for packages, this also breaks anyone depending on them, requiring a fix that is not even in their code. So it's downsides either way, and which one is more disruptive to the ecosystem depends on the proportion of code affected in different ways. I assume that they did look at existing Python code out in the wild to get at least an eyeball estimate of that when making the decision.