I agree with your point, Sony should have annual flagship releases like they have with their other products
But releasing a flagship every 6 months also means that Sony ALWAYS has a top spec device to compete with all other releases on a particular year, maybe that's why they like it
I like regular releases as I work in tech, and get to review them - so it keeps me busy. But I also review things with the consumer in mind, and I do wonder if Sony loses as many sales as it gains - simply because, like it or not, many people don't like spending loads of money on something only to find it is superseded a few months later.
Sure, we all know that there's always something new around the corner. Sure, we know the device will still do everything it originally did before you knew there was a new phone (or laptop, or TV). But, that doesn't change the fact that some people will just prefer to go to Apple, Samsung, HTC etc and have something that is still the top model for as long as possible.
Clearly Sony thinks it's a good strategy and if time proves it with Sony gaining loads of market share because the some people who get upset are less than the some that don't care one bit then I'll shut up, but I do think Apple and Samsung has the right idea.
Still, as long as Sony is releasing a new flagship every six months then I think I'll probably end up getting it - even though common sense tells me to wait for the 'big' update in the autumn, which will have all of the Z2 features, a new SoC, new camera and much more.