If you select "customise" jollyg you can choose how many posts you see on a page - up to 1000. Then you can scroll up or down to see everything posted on that thread.
Or posting could be restricted to X posts before a thread was cut off - but I don't think that would be popular.
As for people wandering off the point, that is how real-life conversations go, and without someone at HQ reading every post before allowing it to appear, or suppressing it if it was off the point, I don't see how you can stop that. Do you really want HQ to censor all posts?
The forums are only a part of the site, however much the posters feel that they are at the centre of it. It costs money to keep a big site operating and interesting to visitors, so income has to be generated, I can't see it surviving without it.