This doesn't address the primary risk that merchants take on: fraud. Either crypto or credit card, if you end up taking a payment via stolen credentials, you will refund the money.
The risk that the middleman in a two-sided takes on is fraud. The scam is pretty simple: steal credit card/crypto key, set up fake seller account, buy stuff from yourself using stolen credit card/crypto key, cash out. The middleman pays the money out to the scammer, and eventually has to pay back the person that was stolen from, credit card or no.
Micro transactions are a huge merchant risk not because angry customers can chargeback, but because one stolen account can undo 1000/X legitimate transaction (where X is your profit per transaction). If your margin is slim, you've just amplified your fraud risk.
And before you say that it's easier to steal credit cards than crypto keys: 1) wait until crypto becomes common and 2) it doesn't matter. If you secure your keys, then scammer finds some user's account credentials and cleans their account out.