Of course it's not rocket science - but it doesn't sound very reasonable either. I don't understand what you mean by "performed its task" - do you mean you buy a completely new system for a new job? Or that the image quality isn't enough for you personally or for your clients?
What's the reason for switching from Mamiya 645, 67 and Hasselblad to PhaseOne and Leica?
I hope you don't think I'm trying to annoy you, I'm just curious!
Oh no I didn't mean you personally, I meant I know a lot of people who cycle their gear continuously and it never seems like they know their gear at all while doing so. I understand now that you really work long with your stuff and then sell it, I'm not sure why but I'm keen to know the reason behind it! 
Hmm that's trues but then again resale value drops as you keep using it, so I wouldn't suggest it's a good idea for a professional photographer to buy his main gear thinking of how much value he'll be able to get back when he's selling it.
I like to pick a system and stay with it as long as possible - only when I have multiple systems at once and I realise that I don't use one I'll sell it. Resale value is nice but I actually learned to prefer used stuff and secondly it limits my choices.
Performed it's task = get the job done to expectation and specification, writing it's value based on $x amount per job, which is worked out
separate to other expenses, rates and fees etc. So for example. 4 jobs per month at $500 per job = $2000 per month, or $24000 per year. So essentially you treat it as though each job is a rental. Once you have made your money back then that is what I mean by "performed it's task". I've never bought a system for one job - I would just hire in that instance.
I listed some of the reasons why the switch around my previous post but just quickly:
GX680 - Incredibly capable but cumbersome. Limiting for what I did, especially since my style went from tripod to handheld.
RZ was incredible and first choice for film, a good balance of size and tremendous IQ. annoying with digital because of the crop and extra wake up cables etc.
645/DF/DF+ - OK but I don't like the lenses, I found af problematic, it's an uninspiring system to me.
V - my favourite MF but limited to P45+ in my experience. When I needed the P65+ it was suddenly a lot more involving with focus and reliability and the lenses were showing their limits.
H - Most efficient and easiest to use. Super bright viewfinder makes for far better focusing with high res backs, better af, superior design, superior build tolerances, but ergonomically challenged at times, suffers with poor connection issues with Phase Backs, don't like the lenses as much as the Zeiss V's.
I guess here is how it is for me:
My first consideration with gear is necessity. eg. If I need the extra resolution then that will win over rendering or ease of use/handling.
Coming a close second is rendering. The way the pictures look.
Third, probably practicality/workflow/ease of use
Fourth most likely how much I like using the camera. ie I love using the M and the V hassy. The H Hassy is OK, the Phase One DF, not very much. I've not tried the XF yet, but it looks good.
Fifth, maybe, or maybe even less significant is resale or long term value. Even if I can get some of the money back, then it's a bonus. But is fun to buy a lens and have it double in value and continue to rise (e.g. Leica 75mm Summilux).
For the past 4 years I have been using 2 systems only. Leica M for almost all of my editorial (thought this is changing) or anything that the small file size will allow and Hassy H/Phase One for Commercial that needs higher res or something I need cropping flexibility with. These two systems have served me exceptionally well and I have no need to do anything drastic, though the Leica S does have quite a lot of allure.