There really aren't many online vendors that accept crypto that I have seen. And if they want to avoid views, the fees for a debit card are lower than most crypto transactions at this point.

>I've seen vendors online offer substantial discounts for using cryptocurrency as payment.

This is actually my 100% use case for crypto. Making legal online purchases at a discount to avoid credit card fees, not retarded speculation or the spectre of crime all the nay-sayers worry about.

Since when are transfer fees smaller for cryptocurrency?

Most online metal bullion vendors. Usually I only buy on the occasions they wave crypto fees; but even when they don't it's virtually always cheaper than the CC/Debit option. Compare CC to crypto price on APMEX for example.