I think you can get away with decent blocks of text as long as its broken up with pictures - gives the reader light at the end of the tunnel and all that
If its a thread that contains no pictures, then yes, I would say fragment it well.
Having written hundreds of mostly gigantic threads over the past 15yrs or whatever, the most changes I've seen are in the responses. The readership level is ever increasing which is fine, but the responses/comments are now mainly a few lines or sometimes only one or two words - completely down to viewing on smart phones. Nobody is going to type a large response on a phone. This can be both good and bad, but back in the day, if a reader was typing on a laptop, then counter technical arguments were often had with links and pdfs posted for further discussion which often meant you could come back with responses to fill in any missing info or gaps that your original post may have been missing.
Nowadays, you kind of have to get all the info the reader might need into the first post, as further conversation may never happen. Which, leads onto ever increasing initial post sizes, and having to think of interesting ways to fragment them without them looking and reading like a sprawling mess!
This is one of the reasons why I always found writing Tech articles in Magazines very different - the information was going to the readers totally blind in terms of feedback, with no way of them ever knowing what they thought of it or if they had any questions afterwards