The existence of wire transfers, cashier's checks, and, you know, CASH undermine your claim that people in payment networks don't desire irreversible transactions.
Basically everybody who's ever sold something in person wants to know they get to keep the money when the other person walks away with their stuff. It's the fraudsters in those everyday transactions who want reversibility.
I am much more likely to buy something if I can pay with a credit card. Making a wire transfer is stressful; I'm worried that I got the wire instructions wrong, and my money will go to the wrong place and be gone forever. Paying with a cashier's check is stressful; I worry that I will get mugged between the bank and wherever I'm spending it, or that I'll just lose it on the way. Cash isn't quite as bad, and I almost always have some cash on me.
But given the choice of paying with cash or credit card, I will almost always choose the credit card. I like the idea that if I walk away from that transaction and then later realize that the merchant sold me something defective or outright fraudulent, I will have some recourse in getting that money back.
I get that a merchant will prefer an irreversible transaction, all else being equal. But I don't think all else is equal; I absolutely buy the GP's argument that purchasers will be more liberal with their spending if they don't have to worry so much about the merchant being a scammer. And regardless, there are many many more purchasers than merchants, and most (if not all) purchasers would at least have a slight preference toward a reversible transaction.