volatility is a matter of market liquidity. you can thank bitcoin's scarcity for all the volatility and slippage!
even if volatility weren't an issue (and it might not be), it was acknowledged early on that bitcoin can't scale to all the world's transactions. this is why hal finney predicted the onset of bitcoin banks, which i think we will eventually see. i hope better decentralized or non-custodial solutions take off as well (lightning, sidechains, etc) but i don't see most of the world transacting on the bitcoin blockchain all the time.