I really think ETH is going to take the number one spot, ETH is slowly growing, it's just a matter of time. Bitcoin is way older than ETH, Only if ETH was older than BTC, we would have been saying a different story now
if bitcoin is getting to more expensive than now and bitcoin is only for exchanging from bitcoin to fiat or altcoin to bitcoin, then maybe eth will take the number one spot for altcoin and eth will replace the function of bitcoin. but we don't know what will happen in future and we only followed the crypto currency world so i think we can waiting that time is come and we can see what will happen. i think its not because eth is younger than bitcoin, but its about the history of cryptocurrency itself.