What purpose does XRP serve in the ripple protocol?
It's not clear how you're imagining doing what Ripple does with Bitcoins, so it's hard to say whether you're correct or not.

If you mean with the Bitcoins still being transferred on the blockchain and being used directly to pay transaction fees for Ripple transactions, that wouldn't work. Bitcoin transactions take time to confirm, and slowing Ripple down to Bitcoin's speed wouldn't make sense.

If you mean with the Bitcoins being virtualized somehow onto Ripple's ledger, it's hard to see how you can do that reliably without a central authority. Also, what do you do with the Bitcoins that pay transaction fees? Destroy them?

You haven't really set out your proposed alternative in much detail. It's possible that you do have some good way to do it. But I don't know of one.

One of the main points of Ripple is to solve the double spend problem without relying on proof of work and without being vulnerable to 51% attacks that have no clear solution. Building on top of Bitcoin would make a system that was at best no stronger.

There are significant benefits to having a native currency on the same ledger.