Ethereum is being overrun by crypto kitties. Basically a killer app that has exposed the low capabilities of the network and has created a massive backlog in transactions. They have put in some small fixes in place but they are bandaids.
More likely the technology to replace both BTC and ETH will be one that can support several orders of magnitude more transactions per second.
I have my eye on BTS (Bitshares) which uses Delegated Proof of Stake and has been sustaining the most transactions per second out of all the currencies. Handling up to 100,000 Transactions per second whereas BTC and ETH are below 50.