Perl's decline was cultural

Perl6/Raku killed Perl.

Python 3 almost killed Python.

It's normal. Once a community loses faith, it's hard to stop them from leaving.

> Python 3 almost killed Python.

People were being crybabies; the critics were extremely vocal and few. Python 3 improved the language in every way and the tooling to upgrade remains unmatched.

Python 3 was a disaster and enterprises were still undertaking pointless 2->3 upgrade projects 10 years later

It was annoying but if it hadn't happened Python would still be struggling with basic things like Unicode.

Organizations struggled with it but they struggle with basically every breaking change. I was on the tooling team that helped an organization handle the transition of about 5 million lines of data science code from python 2.7 to 3.2. We also had to handle other breaking changes like airflow upgrades, spark 2->3, intel->amd->graviton.

At that scale all those changes are a big deal. Heck even the pickle protocol change in Python 3.8 was a big deal for us. I wouldn't characterize the python 2->3 transition as a significantly bigger deal than some of the others. In many ways it was easier because so much hay was made about it there was a lot of knowledge and tooling.

With the benefit of hindsight, though, Python 3 could have been done as a non-breaking upgrade.

Imagine if the same interpreter supported both Python 3 and Python 2. Python 3 code could import a Python 2 module, or vice versa. Codebases could migrate somewhat more incrementally. Python 2 code's idea of a "string" would be bytes, and python 3's idea of a "string" would be unicode, but both can speak the other's language, they just have different names for things, so you can migrate.

> With the benefit of hindsight, though, Python 3 could have been done as a non-breaking upgrade.

Not without enormous and unnecessary pain.

Pain on whose part? There was certainly pain porting all the code that had to be ported to Python 3 so that the Python developers could have an easier time.