Netflix removes content because deals to carry that content expire. This is all footage WWE owns the rights to.
HBO keeps all of their series on HBO GO all the time... it's not like the Sopranos disappears some months.
This won't keep me from subscribing... I'd have been subscribing to 24/7 for years if I'd ever had the option. But it's still a backtracking on the big talk they were doing about the library.
Some stuff does fall off HBO Go from time to time.
Those are movies, though. All of HBO's original series programming remains. They do hold back on some of the sports series, though.
Idk about the movies but for the sports series, that's mostly a footage rights issue. When you start dealing with sports leagues like the NHL or the NFL, they charge a lot of fees for networks like ESPN to use their footage on a digital platform compared to over the air TV.