I was at a conference once were a guest speaker from a large company was talking about project mangament. I dont remember his name but ill always remember what he said.
He said that he would never authorise a project that was expected to last more than 2 years as the longer a project last the more unforseen problems will occur, more project creep will appear and most importantly the original business requirement for the projects existance will have changed radically within two years.
Okay, this two year rule may not be very effective in the aero industry when your talking about a decade just to get the plane off the ground even if your using off the shelf components. But the overall business requirment rule of changes every 2 years is something ive seen time and time again. By the time ive built something people dont want it anymore as its been superceded by events.
The lesson to be learned by this is expect large multiyear projects to always go belly up.