Sellers have benefits going through Paypal as well. As a seller, I can understand wanting to accept crypto as you then not only have complete control over refunds, but the transaction fee is pushed onto the buyer which some in crypto are probably willing to pay because the value has spiked so much from when they mined/purchased it, it seems like free money, but that certainly wouldn't be the case if it went mainstream. As I mentioned above, there are other options for accepting payment outside of Paypal, it's just that once you start looking into them, Paypal starts seeming like not such a bad deal after all. If he is doing large volumes of transactions totaling in the millions of dollars, he'd probably be better off with a proper payment system though. There will be some up front costs but he'd save money in the end.
I ran an ecommerce site doing about that much in payments annually. I was responsible for both the tech and finance/marketing side of things.
We accepted all major cards through Authorize.net, also PP and Amazon Payments.
Firstly, you accept what your target demo has to pay with. You need friction at checkout as low as possible, so many resources have been expended to get them to the finish line. You shunt everyone to any one payment method and you might shed 50% or more of potential shoppers. It's hard to filter those folks out of your sales funnel before they are costing you money in terms of clicks.
Secondly, while you can maybe get a more nuanced set of rates (ie, hammered by Amex, lower Visa fees) they all are
suspiciously similar. Almost like they publicly advertise their rates and tiers for those rates and they all go up at once. It's weird, huh? (no it's not given that the network overlords behind anyone that allows you to accept Visa are the same)
It's not like magic options open up when you scale up to even $15M. Amazon probably can do more to dictate rates, but you know why.
If you aren't in the top 100 or so of merchants your rates are going to suck.
The real business to be in is payment processing, obviously. Selling widgets is hard.