SumUp and Zettle are typically used by small businesses, not big ones, and even in this space, there's competition. [1]
I don't believe MasterCard and Visa are forcing anyone's hand, but they obviously give better deals to card issuers that don't co-brand their card with the domestic payment network. The scheme fee is the smallest part of the overall transaction fee, in the end it's mostly banks that profit from ditching Girocard and the like. These networks cannot survive on their own since people need a payment card that works in neighboring countries.
The EU should really push EPI [2] but I belive the initiative has stalled. Empsa [3] is also promising, but in practice the transaction fees aren't cheaper than using a MasterCard or Visa debit card, at least for now.
1. for instance https://www.stancer.com
2. https://www.epicompany.eu
3. https://empsa.org