Highly optimized and well defined solutions are by definition inflexible. Because they are the result of removing everthing which is not relevant for the task. If you are a company handling thousand of letters per minute, then you don't have any reason to be flexible, because the task does not change between the iterations. I mean we are talking here about industrial level of optimization, not some dude in his small office stamping his three letters a day. Companies on such level optimize everything from highest to lowest levels, because any optimization can make a difference of millions of whatever currency they use.

In that scale, nothing is simple, because everything can become expensive on the high run.