Can’t argue with that. It is easier and more prolonged to simply charge for upgrades for all passengers willing to pay, rather than penalizing parents traveling with children.
If we assume the airlines’ goal is to maximize revenue, then the rest is just crunching the numbers. Air travel isn’t a necessity, per se, but it’s the only reasonable option in many travel situations. In addition, nobody expects it to be pleasant to start with. You expect it to be annoying at best and catastrophic at worst, but to get you from A to B a lot faster than the alternative means.
To me, this suggests that the pool of air travelers who would actually pay significantly to avoid children is small, probably much smaller than the number who *say* they would.
In the case of a restaurant, on the other hand, patronizing any particular one is completely voluntary. Eating out is something one expects to be pleasant, and there is little cost involved in avoiding factors one finds unpleasant. For example, I don’t like eating in places with loud music and/or television on. Customers could choose a no-children restaurant, in theory, without trading off any other value, and people with children have plenty of other options.