If it is not decentralised it defies the whole point of the solution. You then don't need a distributed decentralised network of independent transaction validators aka miners. You don't need any blockchain solution for it all. You can as well stick with MySQL and call it PayPal v2. Paypal also uses a lot of "cryptography" in its solutions but it doesn't make it a cryptocurrency. How many validation nodes are there in ripple? Less than a hundred. Can someone with bad intention and CPU power overtake it? No. It is a payment solution that is using parts of bitcoin ideas for no reason, really, rather than making quick cash on the hype.
Fundamentally I think both XRP and Bitcoin are equally worthless tokens with little practical use. However, XRP at least doesn't waste a huge amount of electricity in the process of sending those tokens around - and it does so instantly. That's a consequence of not being fully decentralized and trustless. Another consequence is that it isn't vulnerable to a 51% attack or a 33% attack.
Of course the market says both of these tokens are worth billions in market cap. It treats it exactly like a cryptocurrency, feeding on the cryptocurrency hype. Clearly the market doesn't value decentralization to the point where XRP is shunned.
Of course it is instant - any centralised payment solution is instant as they live in a a zoo rather than a jungle. That's what I'm saying, Ripple looks like a duck but is not a duck, it has some features of a cryptocurrency but lacks the core one - decentralisation. Also its blockchain is screwed - initial 30k transaction or so are lost. And on a personal level I dislike this company as I was constantly removed from their channels asking why their escrow in 2017 hold more coins than their total supply. They couldn't even get it right. D. Schwartz is probably a decent programmer, and I sort of admire how they became rich by building a lame replica of an expensive watch (bitcoin) - I read all of his posts on bitcoin stackexchange some time ago while learning about the industry...
XRP (or Libra) has decentralization, just not in a form that is accessible to the general public, because that brings in all sorts of other issues.
If you look at how we generally solve problems in the world, the idea that the untrusted general public should be involved as a matter of principle is ludicrous, because that requires significant defenses against bad actors at multiple levels.
Bitcoin didn't even solve this problem, it just raised the economic barrier for bad actors to cause trouble. In practice, many smaller cryptocurrencies are highly vulnerable to 51% attacks, but even Bitcoin mining power is highly centralized to a few companies.
I think the way XRP or Libra solve things (in principle) has very strong arguments going for it, but they are pragmatic, not idealistic. I haven't heard any good arguments on how the way Bitcoin solves things is so beneficial that it becomes a worthwhile tradeoff. I don't see why it should be the defining factor on what is or isn't considered a cryptocurrency.
