I might be able to address specifics if you can give an example of for-profit private project that actually took 6, 7, 8+ years to complete. Gehry tower may be planned to take 6+ years to complete, I have no idea. But, I also don't see any action there. Let's stay out of hypotheticals, such as the Gehry towers. If you think back to projects that take 6+ years to complete they are generally projects that involves government, airports, dams, subway, etc. Theses projects are backed by governments that aren't looking at NPVs with discount rates of 5-8%. Governments take and must take much longer periods of time into account. Private investors want returns now and as high as possible. When your payback period is 6-8 years and your discount rate is 8% things get ugly fast. Not mention the risk associated with periods of 8 years. The odds of catching a market down turn when you project goes on this long is much higher.
Anyway find me a project that took 8 years to complete, was a success and we can pick apart why it's an exception. I suspect it'll be hard to find.