The future struggle for people in these kinds of jobs is to get the wider culture to admit to itself that the latter 4 hours are unnecessary and even detrimental to workers.
I think it's rougher for people to understand that aren't in this industry, and there are so many jobs that _do_ benefit from this in a numerical way, i.e. more parts per hour, more accounts created, sales made, etc.
It's not just software and tech, though. It's almost every 'professional white collar' job.
I work in higher education, and it's exactly the same experience for me. 4-5 hours of productivity a day, at best, and 2-3 hours of just generally wasting time and trying to look busy/meetings/e-mails.
Can't deny that there are some industries where more hours are better and that beyond the 4-5 hour range they don't suffer from diminishing returns. But I'd point to David Graeber here and just say that there seem to be a lot of people out there making themselves look busy for much of the day, and in the process wasting a lot of their lives and needlessly taxing their own emotional states. And together we've tacitly made a decision that this is all OK and normal.
Yup, it's certainly a punchline in seemingly every office job I've ever worked. I've worked a lot of retail and cooking jobs though too, and I think a lot of people come from these fields as well and have trouble distinguishing intellectual work from physical, utilitarian work.
I don't know if they are unnecessary, but they do not require me to be in the office. I usually end up reading up on things, or responding to emails when I am in those latter 4 hours. Those tasks can be done just as effectively at home.