There are two equally important points that need to be brought up here:
On the one hand Nokia is probably justified in not supporting devices which aren't capable of running N-Gage games as well as the latest phones. The N97 is just about to take N-Gage into a very high resolution touchscreen-based age, so it would be very odd to simultaneously make games designed for phones from 2006.
BUT... on the other hand, Nokia did specifically promise that the N73, N93 and N93i would get N-Gage on many official occasions. They even said so in a launch video shown to a large crowd of journalists at the Go Play event in late 2007, which was personally supervised by Nokia's CEO Olli-Pekka Kallasvuo. You cannot get a more direct promise than that. Anyone who owns those phones has a total right to feel hard done by, because Nokia has failed to deliver what they said they would.
What Nokia really should have done was make a public statement the instant they found out that there were problems with N-Gage on the N73, N93 and N93i. But they were totally silent about the issue, and even had an "N73/N93/N93i support coming soon!" message on the N-Gage website's hardware section until just a few weeks ago.
The lessons to be learned by Nokia from this are never to make promises you can't keep, and if you do have to break a promise come clean about it as soon as you can.