How can this scale if millions/billions of customers start using BTC from their wallets stored on their smartphones?
Wouldn't the network get overwhelmed if every wallet/node had to be kept in sync?
it's been answered but to keep it simple
keeping up with the full blockchain 24/7 requires more battery/bandwidth then what is practical today for mobile devices (which many transactions now occur using). 3rd party systems/applications have risen up providing access to the block chain for these devices.
satoshi never envisioned every system playing in the bitcoin marketplace to have a full blockchain feed