Reality is "usability metrics" are often are in negative correlation with "conversion metrics". "usability metrics" are also completely superficial and subjective. Where conversion metrics are goal oriented.
The more you make people able to click away, the more they will. It is hard to know whether that helped the user or hindered him.
It is also true that from many SEO points of view, short bite size content actually harms SEO. Fewer words mean less engagement, less value of content seen by google. It seems to me there are horses and courses. Twitter and facebook are for short bite size pieces pointing to places such as web and video that actually contain the useful stuff. ie the valuable crosslinkages are often outside the site.
There is an old rule, I think applies. It is impossible to make a letter too long, only too boring.
Here for example is an extremely long page that is not a sales letter, but I cannot think it would be improved by separating posts, either from an SEO or a user experience.
It is interesting too.
Even 15 minute videos would need a massive sales letter to cover all the content, and that is well over an hour..
I cannot help but feel, this thread is judging long sales letters on the basis of the worst of their genre.
Frankly - If I am parting with £1000+ I want to know everything there is to know about what I get for my money. If you want to ask for £1000 you cannot do it on the back of a postcard, UNLESS it is a proven brand, and if the brand is not proven , then the sales letter has to provide all the proof, all the detail of what it is, and answer all objections.