The thing about new content is that players tend to go through it faster than a lot of people expect. They will optimize their gameplay to achieve their goal as quickly as possible. If this process is not fun they call it "grind."
It also depends on the content and if it is single player vs multiplayer.
I wouldn't consider myself a hard core player with KSP, but if squad were to add new content in the form of new planets or moons I would be able to consume that content quickly due to my current skill level. When they add new mechanics, such as docking or their new research system, gameplay is altered allowing all players to explore new options. Trillek would be like ksp or minecraft where experienced players would have skills to consume content faster than less experience players. IIRC scott manly (sp?) was able to complete the new research tech tree in less than 10 launches. If the tech tree was scaled to his level then those with less skill would find it too grindy.
In some ways modmakers can fill that gap. We see this with almost any modable game. Mod makers might take the game in a direction that the game devs dont want to go in, but still fills a need part of the community wants. This links into the multiplayer context too. In minecraft the public servers look for ways, through bukkit or forge, to provide a unique experience for players. For this it is up to server owners and mod makers to create new content to make players return.