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?
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.
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.
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.