scalability on its own is not important at all otherwise a lot of altcoins would have been worth $1 million per coin already!
what matters is scalability alongside decentralization. which happens to be the main reason why those altcoins are failing instead of rising in value.
the importance is that we already have payment systems that can handle thousands of transactions per second. example of which is Visa or Paypal,... but we only have one decentralized solution and it remains that way by having certain limitations because it is a sacrifice. you can't gain decentralization and not give up something else.
You make a good statement. Both scalability and decentralization play a vital role in crypto project success but the decentralization or scalability was not the one reason why altcoin didn't worth $1 million per coin or surpass bitcoin market price because the surge in price and the survive level of a coin in the crypto market are measure by the trust/support the community has in a certain project.
The reason why most altcoin failed is because they are highly manipulated so investors invest most in safe coins and we can glare see that in bitcoin, ETH etc despite all the FUDs the coins still come out strong.