ABI vs source compatibility was a business decision for a closed source market. Binary compatibility isn't a simple matter but source compatibility isn't any simpler. I find compiling old software is a lot harder than running it. When software gets old enough, you can just run it in an emulator like palm chose to do. They're different targets for different markets and the N900's market runs Linux where binary compatibility has never been as high a priority as source compatibility. I don't really find one to be a lower difficulty than the other, though.

EDIT: and unless Nokia intends to tell Intel that they can only use ARM, I don't see how there was any choice in this matter to begin with.