I don't see the "greatest threat" of Ripple. It would be like suggesting that Apple the greatest threat to Linux?
Unfortunately the average person walks past the Apple store everyday and knows nothing about Linux, even though Android is sort of closed Linux.
If Ripple becomes the cryptocurrency of the people by being marketed very well, it might take off and become mainstream and destroy Bitcoins value.
That risk seems extraordinarily unlikely. I don't get why folks are worried about it.
1) Ripple is marketed very poorly, Bitcoin marketing is far superior and no one is even being paid to do that.
2) Ripple becoming "mainstream" would be like x400 "beating" SMTP in the marketplace, it isn't going to happen, they have different purposes.
3) They solve different problems, with different solutions, they don't even really compete. Ripple is a debt conveyance system, Bitcoin is an asset class and a payment system. They aren't much in the same game, even though they use some similar technologies.
4) Centralized and proprietary vs open, free, user supported.
5) Ripple is trust based and centralised, Bitcoin is trust free and distributed.
It is more likely that Bitcoin and Ripple will each do better because of each other. Ripple certainly benefits from Bitcoin's leadership in the market place, and Bitcoin can benefit from a trust based system for those that want that. For example if I have trusted business partners who can initiate exchanges.
Your example of the Apple store is an apt example of an innovation that is crossing-over. There are now Android stores popping up.
http://www.wired.com/gadgetlab/2011/12/google-android-store-australia/Android remains the globally dominant phone OS, and is only a fraction of Linux globally even without these stores, but they figured it is worth a try too.
People are more afraid than they need to be. If Bitcoin's value were destroyed, that would hurt Ripple greatly, not help it. Bitcoin is a main reason for Ripple to even exist.