What I've seen with my boosted stories
https://medium.com/@matthewclapham/list/boosted-stories-67e2ba14f4a9
is that the 'should get at least 500 views within 2 weeks' has meant 'will get 500 views, and then that's your lot'.
I'm fine with this so far - it's enough to generate reasonable earnings, and typically to add a couple of dozen followers and a subscriber or two each time.
But it is inevitably disappointing to see it tail off. It would be great for those readers to have a way of passing the boost on by recommending it to their friends/followers with a single click. The question is how to define that mechanic without swamping the system or leaving it open to abuse.