Look, it's not as crazy as you imagine.
Mobile phone technology is increasing at a rate around 3x faster than that of PCs. So, what you're complaining about is analogous to saying that a PC you bought 6 years isn't compatible with Windows Vista or Windows 7.
When the 1st generation Android devices came out (G1, myTouch, etc) they had a very low amount of memory. This is the biggest stumbling block to getting Android 2.X on them.
New phones have better hardware, so Google want to take advantage of that. People would also complain if they bought a PC today that came with Windows 98.
Don't worry too much about fragmentation. Google have stated that they will address the problem of apps needing to be written for different version of Android. It may even be done in 2.2.
Currently, most Android phones are running 1.6. Most apps that I know of that are written for 1.6 work in 2.0. Some apps have specific versions that work better in 2.0 and take advantage of the new features. Most of the 1.6 apps that don't work in 2.0 have a 2.0 version.
Also there are some apps that need 2.0, but tough, that's just like the fact you can't install Office 2007 on Windows 98. Time has moved on, technology has moved on, hardware has moved on, software has moved on.
Unfortunately, it's happening very quickly in the mobile phone world. While this may be disappointing for people stuck on old hardware, your phone can still do everything it could do when you bought it, so you're not losing anything. However, you're coming across as complaining that your phone isn't magically getting more memory and a faster processor every time a new version of Android comes out.