Bull. People developed software for multiple completely different computer architectures throughout the 80s and 90s. People do it today between different game console platforms in addition to operating systems. Even if you try and target something like SDL or a web browser your abstraction won't save you from a platform's quirks once you reach a certain level of complexity and then you'll have to work around it anyway.

Hell, even between supposedly POSIX systems there's a lot of #ifdef going on to make things work.