SnakePromise by that logic, search engines should show you the whole internet on their homepage so you can find what you need with CTRL+F 😛
It's not reasonable to show a full discussion unless you also have a maximum number of posts after which people can no longer post. The number of items per page can be increased, but there will be a point where the performance hit will be too bad.
I don't think you'll find a single discussion software that lets you see a full discussion (with hundreds or thousands of posts) on a single page. Even softwares where the number of results per page is customizable usually have an upper limit.
Usually when you can view/download "all" of something, there will be a delay. For example Google will send you an email when your data export is ready because it'll be put in a queue and you'll need to wait your turn, they can't have everyone view their full search or position history at once. So I can totally imagine a way to export all of a discussion's content "at once", but the ideal implementation for that is asynchronous.
A proper search implementation will also have the benefit of optimizing for what is searchable in the post, as well as potentially searching by keywords that aren't visible in the page themselves (image alt text for example)
