To jump the ship just for that seems unwise to me. There are many other factors. I've done it. no big deal. It is actually quite trivial specially because both sides are so comparable. you give up some, you gain some. When the D900 vs 5Dmk4 game starts 3-4 years from now, I'll revaluate my choice.
Trivial? How many lenses do you own? I have five L's and one non-L. All those carefully researched. I have to sell all of them, deal with unknown people on ebay with all the risks involved, spend a lot of time to research the new system, spend more money to get new lenses (and Nikon lenses are more expensive, even though Canon is trying hard to get a lead); just to discover that my new Nikon body cannot focus as well as, say, my old Canon - for example. And just when I do all this, Canon comes up with a new body, and I start selling lenses again.
I sold over 10 high end canon lenses and two bodies. private sales are best. specially if you find a Nikon shooter who wants to go the other direction. I will not say it is no effort, but considering how much I use the gear, the fact I need to post a classified and deal with paypal is actually a minor issue. I don't do it every month and if the release cycles are any indication, the D900/5Dmk4 won't be around for a LONG time meaning the hassle is a twice in a decade occurrence, at best.
Newer lenses tend to be more expensive. the 70-200 and 24-70 canon revisions are more expensive than just about anything. Yet I fully expect the Nikon refreshes of that to come in about 5 years to reach even higher. Is this not expected? off course it is. It's called inflation, dollar devaluation, and simple feature creep.
I will never marry my gear.
If you're shooting weddings or 'generalist' stuff, maybe. But those gaps in the lineup are significant, and I think Canon has the lead there. The MP-E 65mm for macro shooters and the TS-E 17mm for architecture/interiors are unmatched. For landscape shooters, the Nikon 14-24mm is nice, but Canon has a 14mm prime and a zoom starting at 16mm. For wildlife, Nikon's real and purchasable 200-400/4 is nice, but I'll take my handholdable 600/4 over that any day.
You can do more than *weddings* on a Nikon system. 17mm TS as well as the MP-E65mm are nice yes. And those very specialized shooters will likely not move. In my case I find the 24mm TS to be more suited to me on both systems. But I'll count them as loss for the sake of argument. Landscape wise, the 14mm canon prime which I owned is no match for the 14-24 zoom, in fact no even close. So I count that as a gain. The 1mm difference on the 17-35vs16-35 wide zoom is not holding anybody back. When I switched, I found that Nikon has a 16-35 IS
f/4 zoom that goes down to 16mm vs the non IS
17-40 Canon that is also quite old so that was a gain. I'm much happier with the wide landscape offerings than I ever was before, but I can see how both camps can get hung up on justifications for either side as long as they can makeup some shooting situation. Overall for me, it was a net gain on landscapes even neglecting the sensor advantage. The rest of my switch was more of a flush trade. Nikon has a modern 80-400 which canon absolutely lacks so I gained that and so far have been extremely impressed with it. And as you said the 200-400 which *someday* will ship on the canon side at likely greater cost (not that such zooms remotely peeks my interest for now). My other stuff transitioned easily all the way to 400mm.
600f/4 hand-holdable? I'll take your word for it. You're stronger than me and steadier than me for an 8 1/2 lb lens is not something I'd use without good support but it will save your donkey, I'll give you that
Ultimately, there is not much point justifying our own choices. Going to any system, you gain some, you loose some, which is my point. I certainly see plenty of the same tales in Nikon land. In most cases however it is just the usual nonsensical loyalty photographers have for mere tools which is both hilarious and sad at the same time.
I know that there are people on both sides of the fence stuck on some lens or camera and erect a shrine to it and overblow its significance based on some anecdotal evidence or personal experience. I'm just glad, long gone are the days were it was canon or nothing and we should all celebrate it and hope it only gets to better. The trend certainly indicates the future is very heterogeneous and there will be plenty of choice. AWESOME.