Blockchain scalability while keeping decentralization is now a solved problem, you can research how sharding is implemented in cryptocurrencies such as XTZ or EGLD, or read this rationale [0] for terabyte blocks in Bitcoin Cash. Why do blockchains such as BTC or ETH refuse to scale on-chain then? That's a separate debate, but I believe there are vested interests in them not scaling.

Regarding volatility I agree that it's currently an issue, but not an insurmountable problem in my opinion:

1. Payment gateways can offer automatic asset conversion to minimize volatility risk for payment takers. This means I could pay in whichever cryptocurrency the payment gateway would take and the receiver would get whatever currency they have set up in their account. They might want to keep some currencies and convert others, so the payment gateway could offer an option to decide that, and in which amounts (e.g. "keep 10% of each BTC payment, convert the rest to USD").

2. Price volatility should reduce as a cryptocurrency is more widely used. In the alternate universe where BTC scaled to be larger than all credit card networks combined its price could be more stable than many fiat currencies.

[0] https://blog.vermorel.com/journal/2017/12/17/terabyte-blocks...