But at that point "million line projects, developed by teams of people of varying levels of competence, and with serious reliability and performance requirements (plane shouldn't fall out of the sky)" you also put dev speed and flexiblity firmly into the back burner because other things take priority. And in all honesty in most of these cases it seems like a scripting language of any kind would be out of the question from day one.

The larger the screen (or screens) the harder it gets for me without a "dark" font. Especially if there is not a lot of ambient light, it is easier than having to play with the screen settings several times a day. I will only switch to a brighter theme if there is too much light and the glare gets in the way ( if I am outside or there light enters the window at a specific angle ) For laptops there are also battery considerations to keep in mind keeping the screen brightness high to clearly see the letters but have most of the screen be less bright will reduce the battery drain considerably. But I agree often for people it's more an issue of following the fad more than anything else.