As engines got more efficient, coal became cheaper to put to work, and so people found far more things to do with it. The same thing is happening with developer hours.

Such a strange argument, again. Since the 80s at least plenty of software had a cost of 0. It was available for free and was also free to modify too but required skill. Since then ALL that software remained free. The Internet also made its delivery basically free. We started to have libraries that made the CORE parts of software itself free, e.g. libraries to sort, cryptography, etc. Then we had basically worldwide education on software, including programming language to make it easier, framework to make it easier, software to make software even kids could use. The entire time the cost of software kept on dropping.

What remains expensive... is GOOD novel software.

Not "just" yet another copy of yet another software, e.g. CMS or sorting a list, but novel quality software remains expensive.

Security analysis and monitoring will spread more widely, but will also need to be super cheap. More software means more attack surface.