Bitcoin is going down the way to become the global store of value. The longest data, biggest trust is in Bitcoin. Ethereum is going down the way to become the platform for all smart contract projects.
Still I think if Decred is in the top 20 will mean that it has a multi billion market cap still 100x bigger from now...so very bullish on decred.
Decred is just a cool coin and I think it should be popular and successful. but there are many other coins except Bitcoin, Ethereum and Decred .