1. the decentralization of mining has nothing to do with creation of centralized altcoins such as bitcoin-cash. even if we have the most centralized or most decentralized mining system these types of altcoins can still be created. you can literary create a new bitcoin cash with your own system without even having an ASIC miner. you just have to manually edit the way difficulty works (oops that is exactly what BCH did too
)
when we talk about decentralization of mining we are talking about the system where hashrate is distributed as much as possible.
2. i think your definition of successful coin is broken that you call "XRP" a success!