Why? Is the internet in danger of running out of space? Whether or not it's of value to me or you is irrelevant.
Are irrelevant threads deleted in order to conserve space on the internet then?
For example a thread in the fanfic forum about the pros and cons of eating apples rather than oranges would not create a danger of the internet running out of space, but it would nonetheless be deemed as being 'not fanfic' and subsequently removed.
Post after post of:
2256: a starship leaves the Federation.
2257: a starship comes back.
2258: another starship leaves the Federation.
...and so on isn't fanfic is it?