Knowing that you have F-you money in investments makes it a lot more likely that you can enjoy your job and removes other stresses in life, or at least it did in my case even with only lowercase f-you money.
Paradoxically, I think the willingness and ability to do that (assuming it's used at all reasonably) tends to increase your visibility and perceived value to the company when averaged over a large number of trials.
"This is stupid, but I'm going to keep my mouth shut because I like to eat and live indoors." vs "This is stupid and I'm going to tell the CEO that and suggest a better alternative."
This happened to me sometime between my first and second kids (last year or so). It was some mix of changing priorities, less time, starting to feel my age, work becoming more difficult, company cultural changes, etc. I think I've stuck a decent balance of saving and living, but man does life come at you fast. Went from happily working 60-80 hours (including some side projects) to struggling to work 40 hours. Reminds me of how one day something switched and I just couldn't drink anymore. Age is probably the biggest factor, and I can't imagine what this is going to be like in my 50s (just turned 40).