Ripple and Stellar are not decentralized cryptocurrencies. These are Corporation coins, exposed to the risk of massive monetary inflation.
New altcoin-scams will continue to emerge as long as premine and similar scam techniques will work. The end for most of these coins is clear: They will become untradable and disappear. That's it.
Though it is definitely not a scam. The fact that people actually it might inflate over the long time doesn't mean that it can't be beneficial. Also just look at ethereum, it also doesn't have a cap to which it can be mined, but no one is calling it a scam, then why would we think that ripple is a scam? To be honest I'm putting money into ripple because I believe it will grow