Firstly, thank you for wanting to take an open-minded look into the issue, rather than simply defend a position that you have already committed to.
You write "Why then is it, that we don't all code in Idris, Agda or a similarly strict language?... The answer, of course, is that static typing has a cost and that there is no free lunch."
I take it that you wrote "of course" here through assuming that there must be some objective reason for the choice, and that it depends solely on strictness, but languages don't differ only in their strictness, so choices may be made objectively on the basis of their other differences, and we also know that choices are sometimes made on subjective or extrinsic grounds, such as familiarity. I don't know what proportion of professional programmers are familiar enough with Iris or Agda to be able to judge the value proposition of their strictness, but I would guess that it is rather small.
Now, to look at the sentences I elided in the above quote: "Sure, the graph above is suggestively drawn to taper off, but it's still monotonically increasing. You'd think that this implies more is better." As the graph is speculative, it cannot really be presented as evidence for the proposition you are making. I could just as well speculate that static program checking does not do much for program reliability until you are checking almost every aspect of program behavior, and that simple syntactical type checking is of limited value. That would be consistent with the fact that there is little empirical evidence for the benefit of this sort of checking, and explain why most people aren't motivated to take a close look at Iris or Agda. In this equally-speculative view of things, current language choices don't necessarily represent a global optimization, but might be due to a valley of much more work for little benefit between the status quo and the world of extensive-but-expensive static checking.