I think ETH will be in top 3 cryptocurrency for a long time.
Its always in the Top 3 and it will stay on the second place a long time. Maybe there will be more Valuable Coins, but it will be very hard to create a technical better smart contract plattform like ethereum. Its not impossilbe, and somedays this will happen. But thats a long time, and before this day will come, ethereum will hit 1000$. I think we got the 1000$ next year!