This causes a significant limitation in PHPUnit (and presumably other unit-testing frameworks). It is not possible to mock final classes or methods: <https://phpunit.readthedocs.io/en/8.3/test-doubles.html>. (Mocking a class subclasses it and overrides the methods to be mocked, which doesn't work if they're final.) This makes it difficult to write unit tests for code that uses final classes or methods.

Someone went so far as to write a library that strips "final" from all PHP code to work around this problem: <https://github.com/dg/bypass-finals>

This feature would be useful for MediaWiki, see discussion (we actually want to try using that final-stripping library for test runs to work around the problem):

https://lists.wikimedia.org/pipermail/wikitech-l/2019-August/092432.html
https://phabricator.wikimedia.org/T231419

Reflection allows calling private and protected members because that can be done at runtime. Creating a new class/method that extends a final parent happens at compile-time; the only way to bypass it would be to modify the class data in memory before the child class is loaded, and that is more of a "runkit" operation than a reflection operation.

If you've written a lot of tests then you know that sometimes you have to refactor the source to make testing possible. This is one of those times.