That is an excellent analogy.
Also, there is a reasons why the OS is not kept on a separate storage drive:
1) The device would need to be physically bigger to accommodate that extra drive. But who wants a bigger device?
2) It makes it easier to provide backwards compatibility (i.e. running newer OS versions on older hardware). For example, when you update your OS, the amount of space required to store that OS may change. If the OS was stored on a different drive and an update was released that didn't fit on that drive, you'd have to buy a whole new device with an "OS storage drive" large enough for that update. By having the OS on the same storage drive as everything else, you have much more wiggle room for that update.
(Of course if you've used up all your storage space you'll have to remove some data to complete the update, but that's why Apple recommends to keep a portion of the storage space on the iPhone freed up.)
3) The iPhone is essentially a small touchscreen computer. As such, storage works on the iPhone like it would on any full size computer. If you aren't happy with the storage that is available after you deduct the amount used by the OS, get a higher capacity model.