I don't overly agree. You're falling into the trap of considering people in here to be the norm. The absolute simplest fare structure to understand is one where you require one ticket per person per vehicle journey (like a bus), and where that ticket specifies which train and seat you must use (like a plane). It would not be hard for sales sites to aggregate those tickets in one journey, just like Trainsplit etc already do. Most people book long-distance journeys in advance, I expect there are at least some users of Avanti and LNER who don't even know you can just walk up and buy a ticket, and even more that don't know you can buy a through ticket from Altnabreac to Lympstone Commando if you felt like (though it's a whopping £816.00 for a First Anytime Return via London if you were feeling posh, but don't worry, that includes the couple of quid for the Tube across to Paddington!

)
That doesn't mean to say that that's the right choice to do something like that. But people in here really need to remember that experts who can get the most out of a flexible ticket are very much in the minority, and it's the present system that has issues with complexity, not that one. That one would have issues with
flexibility, but that wasn't the point being made. Do I want this? Not really. Would it make things simpler for the vast majority of users and thus attract more users? Absolutely.
Any simplification, even if it did reduce flexibility, would probably overall do more to get people back on trains (per the thread subject) than complicating things or even leaving them in their present complex form for which the railway has a rather negative reputation.