Say we have a perfect "make illegal states unrepresentable" model. Like what you said, it's kind of inflexible when there are requirement changes. We need to change affected codes before you can even proceed to compile & run.

On the other hand, an untyped system is flexible. Just look at Javascript & Python ecosystem, a function might even contain a somewhat insane and gibberish statement, yet your program might still run but will throw some error at runtime.

Some bugs in programs like games or average webapp don't matter that much. We can fix it later when users report the bug.

While it's probably better to catch whether an user can withdraw a negative balance or not at compile time, as we don't want to introduce "infinite money glitch" bug.