Exactly. Without judging the OP (I am also looking at buying a van (although I would not drive it regularly in the city)) it is a matter of luxury rather than necessity.
I personally doubt the necessity of booster seats for older children at all but that's another topic all together.
The solution to this would realistically be to have built-in booster seats or a different style of seat belt that would better fit children and eliminate the need for booster seats for the older children.