What barrier to entry? Their customers don't care that OSX is running under the hood. You can offer an image processing service using any platform today. Sure, on Linux it probably wouldn't be as efficient, but it doesn't have to be. Scaling is a Good Problem to have.

Basically, as you grow, it helps to take a critical look at risk factors and the technical debt which contributes to that risk. The longer you wait to pare down that debt, the more expensive it is, and the more exposed you are to that risk. A little more work up-front saves a lot of work later on.