In your GP comment, you say "But even aside from that, if you can complete your work in 6 hours, but can't leave the office for another 2-4 hours because of the office culture, then you'll spend those 2-4 hours doing random stuff in the office. If you're at home, you can leave Slack on and go do something useful."
I think I agree with your second paragraph entirely.
To me, two issues are being conflated here: the location of the work (wfh vs office) and the length of the work day. The benefits of wfh you are describing sound like they could more or less also come from a 6 hour work day. Ignoring commute time (it's an issue, but just for simplicity's sake). You'd be able to do useful things around your house just the same if you were in an office for 6 hours and able to go home.
Some would describe what you're saying as "slacking" for those hours outside the six, but I don't see it that way. Instead, wfh has become a socially acceptable vehicle for a shorter work day. It allows workers to achieve a shorter work day while bypassing those arguments. I think productive discussion around the issue is impossible until these issues are disentangled.
this is the core issue - are you hired to do 8 hours of work, or are you hired to complete a set of tasks?
if the former, then yes, you're slacking off if you stop after 6 hours
if the latter, then no, as soon as you're done with those tasks then you have completed your day's work and can go do something else.
old-skool authoritarian management tends towards the former, and tends to measure time-at-desk rather than actual productivity. Hence they don't like WFH because it forces the latter.
The objections to WFH tend to come from managers who manage by insisting their employees do at least 40 hours in the office each week, rather than measuring actual output and productivity, because it's easier. A similar question is: If you're super efficient and get your work done quickly, can you end your day early and go home?