It is realistic. I worked on a cross-platform product for nearly a decade that followed a methodology very similar to this. Our code worked on 16-bit, 32-bit, 64-bit, big-endian and little-endian systems. Following these rules did not slow down development noticeably. In fact it became second nature.
The down-votes on parent are misplaced. Why not enlighten with comments about how it -is- realistic?
Because he/she doesn't explain why it is not realistic, his/her comment doesn't add any value to the conversation.