I've encountered pay machines that only accept cash twice in the past couple of years in rural areas and was surprised. Like you, I thought they were long gone, but I guess there are still some out there. Those ones did have stickers on them (fortunately!) directing me to one of the fleet of pay for parking apps out there, which I used. Not being familiar with the area though, I had no idea if the app I was being sent to was legit. I didn't even think about it, I just thought "oh, yet another parking app I need to download" and went ahead.
Contactless is easy - but there's a far higher cost (each machine needs to have connectivity etc). Plus it doesn't let you extend remotely, which is something I use very regularly. I guess from a user perspective that's a benefit, but from an operator perspective it's a potential downside - would I pre-pay for more parking if I couldn't extend? Probably. But would I potentially park elsewhere, for a car park where you pay on exit, thus removing that revenue from them entirely? Possibly. I guess it would depend on lots of factors, and my one-person study is perhaps not the most reliable data!
I agree that they are becoming ubiquitous because they are cheaper for operators, but I don't think that this is the only reason: I do think that they genuinely offer greater utility than a pay at machine with contactless option.
Interested to know what you mean "monitoring is already done by ANPR" because this is not my experience where I am based. Do you mean street based parking, or in car parks?
For the street based parking where I am the traffic wardens have a tablet that shows them the make, model, registration number of the vehicles parked in a particular area, and generally make their patrols on foot, so I'm not sure there's any ANPR being used. I guess there could be CCTV cameras cover exit and entry points to particular locations, but in European countries and the UK I'm pretty sure that there need to be notification signs that your registration plate is being captured in that situation.