Absolutely agree that the thinking required to write testable code is a huge benefit. And, you don't know how testable it is without the tests.
Enforcement can be a bit tricky -- the key is to get the desired mindset and behavior. Sometimes rules and enforcement can become a monster, and drive behavior that's compliant with the enforcement, without the necessary judgment (or understanding of the right mindset). Maybe an opportunity to start with some lightweight / nonthreatening checks, and view noncompliance as an opportunity to discuss and explore, rather than to reject transactions?
It is important a balance must be achieved for the enforcement. This is why it's up to the teams to decide their own rules.
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