I'm not so sure there's a big market for 6" phablets yet -- that's going into uncharted territory, somewhere not even Samsung dared to go (this year's Note 4 has the same screen size as last year's Note 3, at 5.7"). At some point, phones are simply going to get too big for people, and this phone is going to be very hard to fit in a pocket or use one-handed.
It's also worth pointing out that Google's previous Nexii have had very mixed success sales-wise, so I'm not sure you can really assume that they've got the phone marketing thing down pat, or even that they care about sales that much. With a $650 price tag, the N6 seems more like a developer niche thing to me -- just as the Nexus One, Nexus S and Galaxy Nexus were (which also all had similarly high price tags).
But, I could be wrong ... maybe it'll sell like hotcakes, and we'll be seeing a 7" phone next year.