IMO, having many cryptocurrencies above an arbitrarily-large threshold of USD equivalency (let's say $1b, which according to https://coinmarketcap.com/ right now gives us 40) dilutes the overall chance of cryptocurrencies ever becoming viable for commerce, instead positioning them more similarly to stocks or other speculative investments.
To meet my definition of "succeed" as currencies ("when in actual use or circulation as a medium of exchange"), we would likely need just a few, or likely, one, that works e.g. at brick-and-mortar places. Overstock, Newegg, Steam, etc give good credibility to this idea, but look at which cryptocurrencies they accept (and how the price in those is tied to the real-time market rate).
Also regarding decentralization, and again because these cryptocurrencies largely have value right now only because of their order books on a handful of exchanges, are we actually in a better place? If the feds in the US decided that they wanted to regulate / shut down the big exchanges for whatever reason (e.g. billionaire gets burned and cries to his politician buddies to "fix" it), sure the coins don't go away, but as long as you can't realistically spend them on basic needs like food and shelter, are they actually decentralized? To me, cryptocurrencies are the similar to Chuck E. Cheese tokens that vary thousands of percent in value per year; they have some cash equivalency if you're able to convert them, but alone, they have no real utility.
They decided that it would be a good idea to start centralized, then over time wane themselves out of the "validation" of transactions until it's in the hands of a diverse group of validator nodes where no one person has the ability to change things.
Ripple labs has a very strict way of handling the XRP they hold. It's not just "lose structures" in place, they are using the Ripple blockchain to escrow 55 billion of the XRP and release it on a set schedule of (I believe) 1 billion per month, and any that is left over at the end of the month is re-escrowed. So they can't just "flood the market" at any time, there is a set limit to how much they can even sell without hardforking the blockchain in their favor.
The founders have free reign on their 20 billion XRP, but I don't see why they would ever use it to crash the price.
Profit motive?