What's really sad is that many people don't seem to understand that they even need park reservations. I'm a TA and I can't tell you how many people I've had come to me recently who want to book trips for this summer and have no clue that park reservations are required and some parks unavailable. Heck, even my BIL and his daughter just showed up one day wanting to buy tickets at the gate and were shocked they couldn't. I imagine that someone somewhere has booked a Disney vacation, bought tickets, and then got there with no park reservations.
I don't expect everyone to know all the ins and outs of Disney the way we do on this board.
But I feel Disney is doing everything they possibly can to fully inform guests of what is required to visit. Every step of the way on the website there are reminders and information about park reservations. You cannot buy a ticket without being informed and provided a link to the availability calendar. And then reminded over and over again that both a valid ticket and park reservation are required. Yes, I suppose there are people who walk into your office to try to plan a trip without any prior research, but I know I personally do at least a bit of online information gathering before going to a TA. And in any case, they have you to inform them, so there is that, at least!
As for your BIL, again, even on a day trip, we search where we are going to find out hours,
ticket prices, and other pertinent information. I'm sorry he didn't do that, but I would think the number of people in this day and age who don't do a bit of information gathering before a visit somewhere is small.
If someone arrives to Disney after purchasing tickets, and doesn't have park reservations, I would have trouble mustering a great deal of sympathy, with all the information that Disney gives when you buy tickets.
As for canceling park reservations a guest does not need, I do not think the number of no-shows on any given day (people who book, don't go, and don't cancel) is significant. Certainly not significant enough for Disney to do anything about it.