People will not flock somewhere unless they sense some potential return on investment. If a website looks like it will disappear in a few months, it does not make sense for a user to invest time and effort into it.

You have to either invest a lot to get a critical mass to join your site, or make it extremely entertaining to be there from the start. Apart from all the criticism, this is what Facebook, Instagram, Twitter, and LinkedIn got right from the start. For their intended audiences, it is either useful or fun to be on their platforms.

I don't see much added value for most arXiv extensions, except for SemanticScholar [1], which might have been lucky being one of the first.

[1] https://www.semanticscholar.org/