More revenue is more revenue, even if Apple is as we speak pulling in plenty.
If Apple could target PC users, and offer a low-cost solution, it could work if Apple surveyed users beforehand and asked their views on how such an application should look/feel. It seems most software firms don't do this, since MS' Vista tanked, and it's a huge sunk cost with the R&D and programming involved in creating it in the first place. I reckon that a low-cost solution, which is cleverly marketed to consumers, can eat into Windows' PC domination. Even if MS responds with its own competitor to it, it still would mean better OS' for the end user.