As someone who would like to be working on new, interesting things in 2-3 years rather than bringing old code into conformance with breaking changes, this attitude captures a worrisome trend in development.

On the one hand, it's great that we have platforms that innovate and improve and harden over time, but we're also facing a development culture where more and more time is spent servicing package/platform/language/OS changes that have no material impact on our own otherwise-mature projects.

It's worth being judicious about where breaking changes are applied, right?

We’re not talking about deprecating a feature here, we’re talking about the addition of behaviour that will break existing code, potentially in non-trivial and hard to debug ways, and in ways that could easily introduce security vulnerabilities.

Just bump the major version number from 3 to 4, right? How long could that migration take?

We've done this too many times and we've had enough pain, let's please proceed at a pace where we can worry about delivering our product and not updating formatted strings, thank you.

> treat it as a fatal error

Did you think this through? What would you treat as a fatal error? How would the compiler know if a particular string is old style code wanting to print some characters between curly braces or new style code wanting to string interpolate a variable?

Also see: Python 2 => 3 hell. Nobody wants to repeat that.

Changing something as deeply rooted as the string type?

Python already went through exactly that disaster once before, when they changed the default string type from b””-strings to u””-strings. It took about 20 years for this transition to finally complete.

PHP has also been responsible for the majority of exploited servers and misconfigured applications. Whatever they are doing it, I take it as a strong negative signal.

Python does not do this. A change like that would require a major version number increment and the community would revolt.

Too bad we can't go back in time to 1996 or so.