We've got scalability already in the stable coins if you would consider that. Although, bitcoin remains the coin most crypto enthusiast look up to but, there are phases to the currency and network. One can't rush the other. Its a highly scares commodity and to have its fit value, we don't need to rush it but, allow the laws of demand and supply play Its part in setting the ask and bid price. With a few bitcoins yet to be mined, the issue with scalability and volatility will still remain. My speculations puts me at a point of a scalable BTC when the last bitcoin is mined.
I'm not sure I understand, which stable coins are scalable to you? Even though it'd be mostly pointless since you will not have escaped the traditional monetary system being correlated to the dollar. Most stablecoins just run on the Ethereum mainnet, with that, making them unscalable by default. Transaction times and fees of Ethereum. To me, for a cryptocurrency to reach its goal of global adoption, worldwide instant transactions, anywhere anytime. It should, ofcourse have instant transactions - and second, have extremely low fees >$0.01. Especially needed for less developed countries where sometimes in peak network stress fees can go all the way up to monthly salaries.