I doubt the maximum fare would work for a national railway smartcard! However, it would certainly make people find the time to touch out.
I don't know how such systems work abroad, but there must be a way to resolve that problem?
For a nationwide system, I would also expect that you would buy the ticket and pay up front (from a ticket office/TVM) and it's merely loaded on to your smartcard. Likewise, you could buy a ticket via the web, or a smartphone app, and it's loaded on the smartcard when you touch in - or even from being downloaded over the air to your NFC enabled phone.
In this way, you wouldn't need to touch in or out to be 'valid' and could still be checked by on-train staff and RPIs alike, as now.
For the convenience alone, I'd love to have a single card that contains a number of tickets/railcards - and the PAYG bit would be debiting my credit when paying at a machine BEFORE travel, not after.
You could still allow people to manage their account (card) via a TVM/website/app such that you might be able to upgrade your single ticket to a return within a month, for example.