RPG a Day 2018
3. What gives a game “staying power”?
A broad and/or deep setting, with lots of things that PCs can do and lots of things that the GM can throw at them. My Buffy The Vampire Slayer game The Watch House is easily the longest series I’ve ever run partially through group enthusiasm but on my end it definitely helped that the setting let me drop all kinds of nonsense onto them. Being able to shake it up regularly, including an annual-ish season setup, helped to keep it fresh too.
(This is a similar answer to the 2016 challenge question What makes a successful campaign? - if anything it fits better here. See also long-term (superhero) campaign advice.)
On the player end, it helps to have more stuff to pick up and use for their characters, and chances to use it, although I’m often overwhelmed by too many options. I’ve only run out of interesting new toys in a few long-running games.
(Edit: one of the stars of TWH is doing this too, and namedropped me today.)