The automated test suite can check many code paths but compilation of a statically typed language checks all possible code paths.
Put another way, automated test suites give you an extremely high level of assurance when using a statically typed language. When your test suite passes in the new version Ruby, you're happy but there still could be cases left that you've not dealt with in rarely triggered code paths/conditions.
The problem with this argument is that you never mention the costs or trade-offs of a statically typed language. You presume that you get the benefits for free and I'm certain that is not the case. The worst systems I've ever worked on were ones with complex and poor types and type hierarchies.
What are the costs of a Ruby incompatibility just waiting to be discovered in production ? You can't assume you wrote tests to exercise every possible branch in every method in every object ?
The costs of static typing are reasonable as long as you're not using a fancy dependently typed language. Ask companies that are maintaining long running software -- types help. The investment is there in the beginning, the payoff is over the lifetime of the project.
If types are complex and poorly defined, you can change them ! The compiler will help you evolve your system through type errors. If you have a poorly structured program in a dynamically type language like Ruby then it becomes more difficult to evolve your system fast and with confidence. You're always asking -- have I missed something out ?
I've been writing software for a long time in many different types of languages. I've led many software teams for a long time that use different languages and tech stacks. I have not seen any measurable difference in productivity or defect rate across different languages. I have also looked at all of the research on this and it is inconclusive at best.
Ultimately, I think that choice of language is one of the least significant predictors of outcomes, yet it's one of the most debated and obsessed over.
Edit: I thought it would be helpful to give examples of what I think is more important. Good CI/CD practives, good observability, robust test and staging environments, etc have been far more important in my experience than static vs dynamic language choice.
Put another way, automated test suites give you an extremely high level of assurance when using a statically typed language. When your test suite passes in the new version Ruby, you're happy but there still could be cases left that you've not dealt with in rarely triggered code paths/conditions.