The iWatch will be obsolete in 24 months at most (less if you are the type who values status and want a "$20,000" watch as the next model will probably come out 12-18 months later)
Maybe there are enough uber-rich people that are ok buying $20,000 watches every year, but that seems like a really, really small market.
Then again, maybe the bands will remain the same, so if the primary investment is in the band, then that could last a bit longer. Even then I'd expect that every 2-3 years they will have a form factor switch that will make all previous bands obsolete.
We'll see.