Kinda like how the iPhone 2G can't be upgraded to 4.0, right?
Your correct but there are a dozen of android devices that can't be upgraded Past 1.6 or even 2.0 and some and also the 2g was supported for 3 years so while it's not going to be supported after 4.0 it was supported longer than the g1 which is the orginal android device. While the life cycle of the early devices that can't be upgraded for android is shorter.
And the sw updates are in the hand of the carriers so it leaves you at their mercy. And if you have a device running sense then you have to wait longer than someone who has a non sense device. Sure you can go to xda and download a rom. But since were talking about official releases you have to wait till the vendor and carrier decides to do an OTA