Have you actually used a N9 extensively yet, one that's running the very latest build?
I ask because IIRC, some N950's are actually running builds of Harmattan as old as April.
And that's just all I've seen & read about, there may be N950's with even older builds.

Nokia has already publicly & clearly said numerous times now: "N9 will be supported for years" (plural)
Even Elop is on the record as saying this...

What does this mean? At the very least it means they'll honor the 2yr warranty* properly, by swapping a faulty N9 for a working one.
Of course this also depends on how well the N9 does.
If it's a flop** they may stop manufacturing relatively early, so whatever inventory was left, may have dried-up 2yrs from widespread availability.

They never made the same claims for the N900.
It was aways always publicly acknowledged as an experimental/learning platform.
OPK: "....step 5 of 6..."

They never really tried hard, to shift it towards being a mass-market & consumer-focussed ph by launch.
Primarily because at-the-time, they still felt Symbian was the "golden egg layer".
And they didn't want to risk killing that off, for the sake of an experiment they weren't sure would take-off.

Plus by the time the N900 was ready, they realised they needed to head in a different direction w/Symbian & Maemo.
As now that they were the owners of Qt, they needed to leverage it properly across both platforms.

This is the "biggy", if some major handset maker doesn't step-in soon.
We can kiss good-bye to it getting anywhere near the kind of momentum Android enjoys.

It might still struggle along....
But it'll only be for folk that are prepared to hack their phones heavily on a near constant basis.
Similar in analogy to desktop linux, it's here & can be very awesome, if you've got the time/inclination.
But it also constantly breaks, and requires constant attending to.

*at least it's 2yrs in Oz, think it might only be 1 in the US?
If they ever do get "local stock". Otherwise EU is 2yrs.
**it probably will be, because even it's own supporters believe it will be, thanks to all the uncertainty.
Self fulfilling prophecy to the max...