if you think what certain payment processors are doing with Steam and itch.io is wrong, might it suggest reporting those processors for abusing their market powers to the respective authorities in your country? Japan has already started an investigation, the more countries follow the better

Find the correct authority for every EU country: https://competition-policy.ec.europa.eu/antitrust-and-cartels/european-competition-network/national-competition-authorities_en

For Germany: https://www.bundeskartellamt.de/DE/Aufgaben/Kartelle/Verteiler_Hinweise/Hinweise_Verst%C3%B6%C3%9Fe_node.html

For Austria: https://www.bwb.gv.at/beschwerdeeinbringung

For Switzerland*: https://www.weko.admin.ch/weko/de/home/anzeigen/kontakt.html

For the Netherlands*: https://www.acm.nl/en/tip-off

For France*: https://www.autoritedelaconcurrence.fr/fr/contact-guichet/signaler-une-pratique

* Please double check, as it has no experience with this countries legal system