I never understand the argument that it would be too complicated and cost too much for airlines to sit people where they wanted, unless they charge for the privilege.
Am I the only person old enough to remember when you checked in when you arrived at the airport, and they just filled up the plane from front to back in order of arrival, as people checked in?
You got whatever seats you were given, and because they filled up seats in order, you were pretty much always sat together.
If I book theatre tickets, I always get to choose the exact seats. And that doesn’t cost any extra (OK, I know that will be built into the ticket price somewhere, so I’ll rephrase - there is no discounted price available for not choosing a seat and letting the theatre randomly allocate one for you). So if they can do it, so can airlines.
It isn’t difficult. Or expensive. Airlines have just conned people into thinking it is and therefore that they need to pay for it.
Having said that, if I am bothered about sitting my family together, then yes, I pay the extra to choose seats to guarantee that. It’s not fair to expect other passengers to move for you, if they have paid the extra to reserve their seat.