It wouldn't surprise me if the iPhone 5 was the last iPhone to carry a number. The name iPhone 5 resonates with me, as does the name iPhone 4 and iPhone 3GS. Each of the names have a natural flow to them, and don't take any time to get used to. The iPhone 6? Yeah, not feeling it.
I can't say I've ever been displeased with Apple's product naming, so I'm confident that when the time comes, they'll change the course of the iPhone's naming and either a.) come up with a newer, better name, or b.) simply stop adding a number to the end of each new iPhone.
At the end of the day, though, the product itself is infinitely more important. I might not be jumping in joy when telling my friend I got the new iPhone 15, but hey, if it's a good phone, it's a good phone.