They are mutually exclusive statements.
When quoted out of context I would agree.
The ticket combinations
aren't that complicated. An open (or flexible) ticket is flexible, and you should assume that discounted (advance) tickets or non-flexible ones have conditions attached. Likewise peak and off-peak tickets.
If in doubt, assume the worst and if necessary ask someone for help. Then you're not likely to get caught out.
That is, to me, pretty simple.
I personally don't buy fully open tickets as they cost a fortune, but given I know they're expensive because they're flexible, it does kind of point to the fact that maybe those extra cheap ones aren't.
The same with flights. I've never bought a flexible ticket, but my wife has for when she was studying abroad and didn't know when she was going to return. She'd have actually been better off just buying a cheap Ryanair flight when she needed than having this expensive ticket, but I can see why you might want the reassurance of having it ready.
As said above, many TOCs allow the sale of tickets onboard. In fact, any staffed train is likely to do so. Those who are driver only clearly can't, so they have penalty fares instead. The alternative would be gates at every station, operating all the times trains are running, or revenue on each train (3 if you had a 12-car train with no walkways) and I suspect fares would rise considerably, or stations would have opening hours slashed (and then no trains calling there) or closed completely.
I am not actually against the idea of having staffed stations until the last train, for what it's worth, but do accept the cost would be very high. You'd still get people trying it on though.