How bringing features like NFC or multi-core support to current Lumias help anyone, if hardware pieces are missing?

Of course nobody expects that the existing phones will magically grow new hardware :-) But there are plenty of software-only features like native code applications, better multitasking, sensible VoIP support, or a more modern browser. Some of these are filling in really major gaps.

Also if the current phones can't be upgraded, it seems virtually guaranteed that Nokia will continue to release new low end devices running WP7 for a long time. This means that those phones couldn't e.g. support NFC, and would be rather unappealing devices if NFC finally takes off.

I'd rather wait with jumping to conclusions about feature set of WP7.8. It's still not released yet, is it?

Having 7.8 doesn't make them look any less dead-end phones.

Would you buy a new Android phone that can't be upgraded to ICS? Would you buy a new iPhone that cannot be upgraded to iOS 6?

I bought Lumia 900 the day it was released and no, I don't feel it's a "dead-end phone". Given that Microsoft backports many of software features of WP8 to WP7.8 (which BTW was under big question in Lumia 900 launch time frame), I'm even less inclined to think so. Will I be getting WP8 device any time soon, just because it's there? I don't think so - I'm pretty happy with my phone.

Unfortunately I think your kidding yourself.

Native applications will not be making it to the Lumia 900 and as such your going to start seeing more and more apps (and especially games), not be able to run on the phone.

It's OK, I think I could survive that.

Isn't this true about applications/games with dependence on hardware capabilities also? How's this different?

This is different in that it's nothing to do with hardware. A super simple contrived example would be if someone builds a calculator using c++, you can't install it even though there is no hardware reason that you couldn't

Look, no matter what phone or OS is, there will be something that you won't be able to run unless you want to update every piece of hardware/software every few months. There's always a trade-off if you are not trying to be "current" at all costs. It's OK, there're more important things in life than an ability to run a piece of software on the phone.

How many WP8 features will be eventually backported? Will there be a 7.9? 7.10?

It's a current phone, much like my Macbook is a fairly current (as in "non-vintage") Lion-running Mac. Like my Macbook, it won't run the next major version. The difference is that my Macbook is 6 years old.

What about third party support? Given how few WP7 devices were sold I could see support being dropped very quickly. That would be my biggest concern

It's been said here several times: Buy a phone for what it does today, not what the manufacturer or carrier promises what it will do later.

You know that a lot of people do buy a new Android phone that can't be upgraded to ICS. Don't you?