This somewhat annoys me. "Knowing what you are doing" is time efficient. Regardless of what tooling you have.
(Granted, the most ported and maintained software I know of is either Linux or SCUMM.)
You combine the potential for side effects in the code with a tree that can't easily be changed because of how many other things it will negatively affect and you end up with a code base that is harder and harder to change the larger it gets.
In a FP approach, this is a class of problems that you can't easily replicate. Yes there's a learning curve to operating in a FP style, but once you're able to move quickly with it the long term rewards are a byproduct.
The OO solution to avoiding this problem is to build with microservices instead, which forces different parts of the system to be isolated and minimizes the negative effects from entanglement...but microservices come with their own maintenance and speed of development headaches as well.
I would love to know that there is a panacea that automatically improves productivity. Unfortunately, I haven't seen one in my career, as yet.
I am certainly more happy in many functional languages. And I can see happiness contributing to some. But I can't count that as data, without RCTs or something similar.
I'm not going to go so far as to claim that computer science isn't science. But I do fear that there are more claims than there are checks in our industry.
I suspect you've seen many of them. They're just the ones that are so normal you don't notice anymore. For instance, no one does pure waterfall-style project management anymore. "goto" programming has completely dissappeared. Etc.