Interesting. In games like Martin's Dungeon Bash and Dredmor the only way to "win" (i.e. live) is not to play.
Endgame is important to me in two major ways.
A time comes to tell the player "you have won, go maybe live your life". I think open-ended games have too much potential to entice a player into boring themselves.
Secondly, if death does not get the character sooner this is another way to giving option for fresh start and possibly trying other options, creating a character that adopts another set of traits, exploring another playstyle, trying some optional challenge.
Finally, when you have already won a game playing it afterwards feels different. I am eager to take more risks and explore tougher areas. In DCSS I stayed safely away from Tomb with my first winner character. The second who got powerful went there and finally died at the top but was close to getting at the rune. The run was very exciting though. If I had not won Crawl before I probably would feel some regret over the death of this warrior.
Endgame is not about lack of content in my case. Not enough fun stuff is likely prompt me to reduce number of levels in game as it already happened once in PRIME. Percentage-wise the most levels were cut from the last part of game - the mainframe. Either the player is tough enough to survive there without taking a scratch or will be dealt with by any two daemons teamed up.
