Do you have any specific APIs or features in mind?
I think most app devs look at the installed base numbers and choose to support 2.1+ or 2.2+. I can't think of any APIs in 2.3 or 3.0 (or even 4.0) that are so wonderful that they are worth cutting off such a large part of the potential customer base.
So in that case, what about customers who DO buy new devices? Your argument suggests that developers won't build software to target the new capabilities their devices have. In which case the manufacturers' refusal to keep their old devices up to date is actually undermining their ability to sell new hardware ("Why should I buy a new android phone; all the software is written to target my existing two year old model")
There are many ways to make a new device attractive besides making it incompatible with old devices. Support for new radio standards. Larger, brighter, higher resolution screens. More flash storage, a better camera, more attractive industrial design, more built-in apps, and a more polished UI for built-in apps.
Will this sort of backwards-compatible improvement always be possible? Probably not. Someday all the specs will be maxed out. But for now it's a good strategy.