Your homegrown framework is only as flexible as it is testable.
If you can unit test that framework without craziness, it’s flexible.
If you can’t, it’s not.
Even if I accepted that TDD and Unit tests were bunk for all other reasons, the fact that they result in a flexible design makes up for any other perceived flaws.
It also cuts through the bullshit. The next time someone tells you their code is flexible, ask them to write unit tests for it.