Bitcoin's dominance can definitely fall to 20% or less in 2018, though it's quite unlikely as bitcoin still takes all the media publicity.
This is a lot more likely to happen if the blockchain stays congested for longer. We desperately need lightning network ASAP. The core developers definitely need to work fast. We're still sitting on 156090 Unconfirmed Transactions[1], which is quite lower than it was a week ago, but is still significantly high.

[1] https://blockchain.info/unconfirmed-transactions

Edit: correction. Thanks to Samarkand for pointing it out.