To those who say Bitcoin and Bitcoin only, what would you say to those who notice the dominance share of Bitcoin declining over time relative to the crypto market as a whole?
Is this a temporary trend?  Immaterial?  The numbers seem hard to ignore at present. 

With how quickly the space is moving, it's certainly possible that the eventual winner(s) in the space won't necessarily be those at the top of the list right now.  However, it's hard to imagine BTC / ETH, at the very least, being entirely supplanted.