I will answer about the banks side, as I know this well. Most of the banks handle the infrastructure in such a way that while it needs to be very reliable and operational 24/7 it's also sized usually for normal operations (plus contingency) because infrastructure operational costs are quite big (and even if you can externalise some of it, there are banking laws that limit that). So any big increase in the number of cashless transactions means investments in the banking infrastructure.
These investments don't scale linearly with the number of transactions. The largest part of the bill comes from upfront software development and ongoing maintenance of your good old 70ties/80ties COBOL mainframe, as well as its integration with newer systems like online banking, etc.
The thing is that once this system is in place, scaling it up or down with the number of transactions is easier and cheaper. Transaction processing isn't the most resource taxing thing in the world anyway. It has to be reliable from a purely functional point of view, but that is true whether you have 100.000 transactions per day or 10.000.000 transactions per day.
More importantly, the knowhow how to make and manage such systems is readily available for purchase.
The big barrier is convincing small businesses to invest in modern POS terminals, to train them to use them and then to force consumers to use cards. Just this part is quite expensive, but more importantly it simply takes a lot of time. You can't really force this change either. You will likely cause huge disruptions in the economy, on par with the current capital controls which translate in MASSIVE costs.
To be honest, I can't take that point seriously, when at the same time many people claim that Greece is being subjected to woes similar to the Versailles treaty. Humanity has surely dealt with more demanding and challenging tasks than training people how to use a credit card.
Especially when they go through some hardship. If somebody would tell me that you can't implement cashless payments because people just don't want to be trained on using them, I'd rather take that as strong evidence that these people just don't want to change anything period.
Of course you need some time to change the way people pay, but that doesn't mean that you should not do it.