Have you tried measuring how many of your altcoin payout customers hold rather than exchanging for bitcoins?

Obviously this is impossible to measure for one of your altcoins, but it should be doable for the ones that have blockchains.

While Ethereum will never be implemented as a sidechain of bitcoin

There's a company that's begun doing just that: http://www.rootstock.io/

I believe the video was uploaded to YouTube a day before the Braintree announcement.