The sad pathetic truth is, that in the digital age in which generations of humans have been conditioned into watching videos, TV Shows and movies where another persons imagination is used to share ideas, limiting the viewers own ability to imagine and create their own images, people get bored reading.
These are the kinds of people that prefer others to do their thinking for them, tell them what the point is, provide one-sided evidence and lead them to a conclusion without showing the other side or version of events.
So when you write in depth lengthy articles you lose these slow, narrow minded dolts as they're incapable of focusing their attention on anything that requires they use their own brains.
Peachpurple, proves my point in the comment below admitting to seeing a 3000 word article frightens these readers and they get bored after reading the first paragraph.
I would say any article longer than 3,000 words should be broken into two articles but with only a related title, do not use part II, make the second article stand on its own.