If a vendor wants $10 per day profit from 10qty of goods, he'll simply attach $1 profit on each of the goods making the total profit $10.
Now, if the QTY of his goods he sell increases to $100 per day, he should be able to sell at a cheaper rate. Making the profit on each goods $0.1, or if he wishes to maintain the $1 profit, he would still make much profit.
Why does the BTC model differ? If there is a congestion in the network, the transaction fee becomes high. Does it therefore means that, if BTC is massively adopted, the transaction fee will be outrageous?
The BTC fee model differs from what you describe in that it's not influenced by the economy of scales, but rather by supply and demand. There's a limited supply of available space per block, so if the demand increases (ie. more transactions to be sent), the fee rises accordingly.
Hence the importance of moving onto 2nd layer solutions such as Lightning Network for day-to-day transactions, as that way Bitcoin can scale massively while largely reducing the fees per transaction in two ways: (1) reduced demand for on-chain transactions leading to lower on-chain fees and (2) being able to do a large amount of transactions at just the cost of two (ie. opening and closing the channel).