I do not own any of the lenses discussed in this thread, but I think that the confusion you experience on here has more to do with what you expect from the lens, rather then the lens's actual performance.
Who should buy expensive prime?
Anybody who wants it and can afford it.
Who should buy 200 F2.0 over 70-200 IS II? Anybody who:
- wants it and can afford it
- is sure that will not miss versatility of a zoom or has/can afford more good primes to cover longer/shorter FLs
- is prepared to keep switching lenses if needed
- wants larger aperture.
Why 200mm f/2 is so much more expensive than 70-200?
-primes are generally more expensive than zooms and we all now that and we all know why. You also pay more for larger aperture and better IS. But just because it's 3 times more expensive than 70-200, does no mean that it will give you 3 times better IQ. You will probably be able to see slightly better performance and general difference in picture at 200mm comparing these 2 lenses, but I believe the reason why not so many people here are pushing you to pull the trigger on it, is, that for what you intend to use this lens for (family, etc), 70-200 is simply more suitable, with really marginal difference in IQ. You don't seem to care about the lack of reach anymore, but even if you did, 300mm would probably result in similar responses. Nobody is saying 200mm is a bad lens or overpriced for what it can do and sure, if you really want it, get it and it will be amazing, but I'll bet, that for those family pics on the beach you will still be using your 70-200 more often. And that's the whole point.