As an example, many real-time systems are often a giant ball of messy asynchronous code and state machines. Futures can help with that, although lately I have found that somtimes the best, cleanest, way to implement a state machine is to make it explicit.

How much do you attribute that to the benefit of creating a high barrier to entry for modifying that code? Could this be summarized as: code that inexperienced devs can't understand, stays performant because they can't figure out how to change it?

None of the teams I've worked with had such a policy and certainly I wouldn't work in a team like that.