I think that altcoins are already very important in the crypto world. Take for example the same Ethereum. As you rightly pointed out, alternative cryptocurrencies compete with Bitcoin and thus make it develop. It's hard to say whether a currency will be more important than the BTC. Bitcoin price is high everybody cant have enough money to buy bitcoin their first choice is altcoin or token. Sometimes they work as bounties so that he can earn some token and later he can exchanged with bitcoin.
Everybodys target is bitcoin thats way bitcoin price is high, Its quite impossible to replace altcoin instead of bitcoin. Do you think any mobile can replace instead of Iphone? It quite impossible. There has many mobile in the market but Iphone is exceptional thats why it took the market. Like this Bitcoin is the pioneer and till now have in leading position in cryptocurrency market, Last 9 years nobody can beat him. Altcoin would be as important but it would take time.