I don't think Ethereum, ripple or any other alt-coin will take over the market cap of Bitcoin, or Volume (USD) traded because these things do not compare to bitcoin in terms of security or recognition. If an alt-coins market cap does end up surpassing Bitcoin, I would theorize this would be the "bubble" everyone likes to pretend Bitcoin is, and the artificial market cap will be shaved off in no time. I only think ETH or Ripple can be artificially "more valuable" if it ever even comes close.

These secondary coins are only thought of or bought at all because of what bitcoin has become and what others know about bitcoin; none of these other coins stand on their own two-feet in the long-term. Bitcoin will keep its seat until it is not applicable, irrelevant or refuses to develop solutions to certain problems.

I don't think that any coin created in bitcoins image, as a copy, except slightly different (normally these differences centralize distribution, development, or allow for other flaws/faults that are not possible with bitcoin); they lack the foundational principals that have made Bitcoin the integral giant it has become in our world of modern finance.