How expensive bitcoin blockchain is, is relative. While it would be unfeasible, or rather not the most economical way to pay a cup of coffee when fees can be in the range of 1–3$ per transaction (depending on your transaction, how time sensitive your transaction is and the how much transactions there are in the memory pool). But that 1–3$ could be the same if you send 1$ or 1000$ or million dollars worth of bitcoin. Because the fee is calculated from the size of the transaction, not from the value being transacted.
Now because the fee is not based on the value of the transaction compared to VISA, it can be a lot cheaper for all parties to pay using bitcoin instead of credit card where it possible both parties end up paying the card company percentages. So say there’s a cost of 1% for the customer, from a 1000$ payment that’s already 10$.
about the volume of transactions… I’m not sure best metric to use is transactions per second, because that number really doesn’t translate well. if there are 100 people transacting, it doesn’t mean it’s first in first out, processed in batches of 3 per second. They might all be included in the same block and that block might be 10 minutes away no matter if 1 or 100 people were transacting at the same time.
Also if you’re selling coffee or other low value services or goods and accepting bitcoin as a payment, I don’t think you would care that much if some of the transactions are never included or if they are included in an hour. It’s quite feasible to accept payment when the transaction is visible in the memory pool and that can easily happen in matter of seconds, instead of minutes of hours.
One could also argue that most of the time the merchant would actually receive their funds sooner with bitcoin than using VISA. With bitcoin, when the transaction is in the blockchain, the funds are spendable by the recipient, while with VISA it’s just an agreement that some day in the future, this amount (minus the fees and such) will be payed to your account.
