This is of course an opinion, but I've been following Ripple pretty closely. It's taking a much different approach than BTC or Ethereum in the sense that its getting the banks involved, and operates using a "web of trust". This results in much quicker transactions.

You can also convert between currencies, assuming you can trust the third party. This isn't that much of a stretch when you consider that we already do it in markets like Coinbase's exchange (GDAX).

They market themselves as the "bridge currency" in the sense that you in addition to sending money, it can easily bridge between multiple currencies. USD sort of fits that niche at the current moment, when you look at forex markets, most currencies (with some exceptions) are compared against USD.

I could see Ethereum overtaking bitcoin since it can be seen as a more functional alternative.

I could also see it not if this pump and dump ICO trend grows to the point that if defines ethereum the way darknet markets did for bitcoin.