Let me rephrase the question a bit, because at extremes everything is dangerous. Is it any more or less dangerous, on average, than other things in society considered to be bad? Are there more or less hikikomoris than DWIs? Are there more or less WoW addicts than rich old men with boats or other (presumably harmless) unproductive activities?
> If an employee spends 8 hours browsing Reddit, instead of doing work that needed to be done, the loss of productivity is quantifiable
Are you sure? Did they really lose 8 hours worth of work? How is "time spent" any better a metric for productivity than counting lines of code?
I find, for me personally, that when I am burnt out I won't produce in any amount of time until I recharge, and when I'm fully charged I'm not interested in browsing Reddit. This is a personal anecdote, and not data; but can we get some data--any data--correlating productivity and browsing Reddit, positively or negatively? I suspect the relationship is a lot more complex than "X hours lost."