Have you got any actual examples of backward compatibility problems on the Mac? I've written a few Mac apps that targeted Jaguar, and they still work fine under Leopard.
Also, Apple never just pulls the plug on an API - at worst, if you use an API, and then Apple decides to remove the API, the next version of the OS simply lists the API as deprecated, and you get a warning when you compile. It is only in the following version of the OS that the API may actually be pulled. I don't think it's unreasonable to expect developers to update their apps every 2-3 years if they want them to keep working.
Also, Apple never just pulls the plug on an API - at worst, if you use an API, and then Apple decides to remove the API, the next version of the OS simply lists the API as deprecated, and you get a warning when you compile. It is only in the following version of the OS that the API may actually be pulled. I don't think it's unreasonable to expect developers to update their apps every 2-3 years if they want them to keep working.