The big difference with a restaurant, though Farago is that there is a constant cycling of availability in the restaurant - people who arrived early finish their meal and leave, and their table becomes available - so if a group arrives, and cannot be accomodated together immediately, then there's the possibility of them being accomodated later on, when other diners have left.
But on a plane, you all get on together, and all leave together, which makes the whole seat allocation thing very different.
Whenever we've flown with the dses, we have factored in either paying for seat allocations or for priority boarding, to the cost of the tickets. I think if you go into it with the knowledge that the advertised price is only a fraction of the actual cost, that there is a menu of extras that used to be included in the ticket price, but that now you have the option to decide which ones you want and which you can do without, and if accept that you are going to be paying a big whack more, once you have added on the extras that you choose, then you are less likely to be disappointed or frustrated.
To go back to the restaurant analogy - we used to have a table d'hote menu situation - you bought a ticket, and it entitled you to a baggage allowance, meal/drink etc - now you have an a la carte situation. Both have their merits - on one hand, for some people, it is easy to plan the costs of their holiday when they know that the ticket price includes all the things they want from their flight. On the other hand, some people like the option to have a cheaper flight by doing without some of the things that the all-in ticket price includes. If you can manage with just cabin baggage, you can save the cost of putting a case in the hold, for example, or if you don't mind where you sit, you can decide not to pay for pre-allocated seating or priority booking.
But if you decide to go for the cheaper option, it is naive to expect to get for free, all the benefits that people who have paid more are getting - you could compare it to going to the restaurant, paying for just a starter, and then insisting that you should be given a main course and dessert too, because those people over there have got a main course and dessert (because they have paid for them either via the a la carte or table d'hote options).
On threads like these, you often see stories of parents arriving on the aircraft and acting in a rude and entitled fashion, demanding that people who have paid to book a particular seat, give it up so that they can have it without having paid for it - that would piss me off mightily, and I would be very likely to refuse to move - not because I am a nasty person, but because I would resent the implication that I should cheerfully hand over however many quids-worth of prebooked seat without a please or a thank you - and without any guarantee that I would be reimbursed or that I would get a seat of similar quality to the one I had paid to book.