I'm not sure or not if the rules stipulate that if you have a card, you must use it (if the facility is available), or face a penalty fare / prosecution.
That would simply be a non-starter in my opinion.
I always thought that the rule was that if there was a ticket office you had to use it (or were at best getting an undiscounted single/return onboard), but if it was just a machine, you could but didn't have to, so could by all tickets onboard. Though not sure if that's a TOC or national rule.
Another problem where some stations lack machines or offices, is that if it's busy and the guard doesn't get round to you, the ticket selling facilities provided before the barrier at termini are often limited with long queues. Which I understand as a disincentive to chancers, but it's not great for those legitimately travelling from unstaffed stations.
I think the rules are pretty clear: if there's an opportunity to buy the ticket you wish to purchase before you board then you must do so. Failure to do so, in a penalty fare area, leaves you liable to a penalty fare.
By definition, if you have a card, there's a card only machine, and the machine is both working and sells the ticket you're after, you have to use it because it is an opportunity to purchase a ticket. If you don't, you're liable for a penalty (in a designated area, obviously).
I agree that this is a huge change for many from how it's been for a lot of years. I agree that it's been poorly implemented. But, as I read them, them's the rules.