Also, have a look at these threads:
That was was purchased at 89,000 miles on it. Pressure check turned out fine, so the ADVer did some basic things to it and started riding it.
Also, have a look at Pyndon's thread: http://advrider.com/forums/showthread.php?t=147440
Pyndon eventually had to rebuild his because the engine started leaking where the cylinder mates to the case. A few bikes have seen the same thing. His post at #1134 shows that the leak started at nearly 70,000 miles. His rebuild showed that the engine had very, very little wear internally.
You can also see Aurel's post, where he and a friend traveled the world on 990s: http://www.advrider.com/forums/showthread.php?t=564699
So, can they last? You bet. Like anything, you have to take care of it and address potential issues before they become problems. But yes, they can last.