Maybe theymos can enable this (again?) since long page of texts isn't much of a resource hog.
Wall Observer disagrees 
Clearly, there should be a limit, but I too often miss this feature for long threads."Show all" is so convenient to CTRL-F something. I usually end up scraping the thread (or someone's post history) to search the raw text.
Update: it won't give you a printer-friendly format, but I made this for searching big threads: "Show All" on long topics.