They only use cards because credit cards can allow them to get sales from people who wouldn't have been able to buy the product through debit itself, but they can buy it from credit.., so they are okay with eating 1-2% of costs in the fact that sale might happen and the companies will get 0.5-1% of it to you back as rewards (hopefully) and so there is incentive to buy using credit card for rewards but they might also give incentives of 1-2% if you buy through cash since they aren't eating that 1-2% cost.

And this whole network has now been built in such a way that now even debit costs the same charge just as network fees

Open sourcing this might be a step in good faith and I mean, we have UPI where I live and it has 0 fees and trust me its crazy good. I personally wish that either everybody in the world could use UPI or pixis from brazil.

The argument doesn't hold - if cards were only about getting sales which they otherwise wouldn't get - and the part about "getting sales which they otherwise wouldn't get", is true enough - then there's no reason to refuse cash payments. That's additional sales, right? But the fact is that more and more shops refuse cash payments entirely. "Pay by card or go somewhere else".