React also went through a lot of churn. (Still does.) There's no magic optimal duration for keeping API stability. Not in general and not for specific projects.
Ecosystems sometimes undergo a phase-shift. Sometimes they take a long time, based on the size. Python 3 was released in 2008, just a year before Angular 1. And the last Py2 release was in 2020, about 2-3 years before the last AngularJS version. (And of course there are many businesses running on py2 still. I know at least one.) These things take plenty of time.
Angular1 was pretty opinionated, willing to break with the tradition of just add one more jQuery plugin.
Miško was working at Google, he persuaded some people to take a look at the framework that he and Adam Abrons were tinkering with.
Angular 2 was announced in 2014 January. And then v1 still got years of support, even the component architecture was "backported" around 1.5 (in 2016?)
You can run old v1 code side-by-side in a v2+ app up until v17. (At least the v17 docs describe the process in full and later docs link to this page. https://v17.angular.io/guide/upgrade )
...
Google did a pretty good job IMHO. Google throws products under the bus, but not so much OSS projects. (Though the sate of AOSP comes to mind.)
